it will be convenient to further describe the present invention with respective to the accompanying drawings which illustrate possible arrangements of the invention . other arrangements of the invention are possible and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the proceeding description of the invention . fig1 is an isometric view of the hoist mechanism according to one embodiment of the present invention ; fig2 is a schematic view of the operation of the single hoist according to another embodiment of the present invention ; fig3 is a schematic elevation view of the trimming cylinders according to another embodiment of the present invention ; fig4 is an isometric view of the split trolley according to another embodiment of the present invention ; fig5 is an isometric view of the duel spreader assembly according to an embodiment of the present invention . fig6 is an elevation view of the hoist mechanism according to a further embodiment of the present invention . fig7 is an isometric view of the trimming cylinders according to a further embodiment of the present invention . fig1 shows the hoist mechanism 5 mountable within a fixed structure ( not shown ). in broad terms , the hoist mechanism 5 according to this embodiment , comprises a single hoist drum 100 , a split trolley 200 , a duel spreader assembly 300 and an end block assembly 400 . cables running continuously from the hoist drum 100 to the end block assembly 400 pass through the split trolley 200 and are directed downwards to engage the spreader assembly 300 and terminate at the end block assembly 400 . thus , rotation of the hoist drum 100 , with the end assembly 400 being essentially fixed , will cause the duel spreader assembly 300 to lift or lower a load attached to this spreader assembly 300 . fig2 shows a schematic view of the operation of the single hoist 100 . in this embodiment , the single hoist comprises a single drum 105 to which cables 10 a , c for the first carrier assembly 110 a are attached . further , a second cable set 10 b , d also are attached to the single hoist drum 105 and engage the second carrier assembly 110 b . both cable sets 10 a , b and 10 b , d terminate at the end block assembly 400 , which provides a substantially fixed point . hoist systems of the prior art , for a duel carrier arrangement , will provide two hoist drums , each operating with a single carrier assembly . thus lifting and lowering of the freight devices attached to the carrier assemblies require synchronization of the two hoist drums to ensure the containers are lifted and lowered at the same rate and from the same position . whilst a small differential may be accommodated , it is still a technically difficult exercise to synchronize these distinct hoists . this is where the single hoist system according to one aspect of the present invention is of particular advantage . for the manipulation of two or more freight devices , it follows that the loads will travel at the same rate , both vertically and horizontally , and so permit the use of a single hoist drum 105 . by connecting two sets of cables 10 a , c and 10 b , d , rotation 125 of the single hoist drum 105 will operate both cable sets 10 a , c and 10 b , d simultaneously , which will respectively cause both of the carrier assemblies 110 a , b , to move 130 a , b at the same rate . thus , so long as the pick - up point of the freight devices for each of the carrier assemblies 110 a and 110 b is the same , then it follows that there will be little or no differential between the position of the containers when they are lifted and lowered . fig1 and 3 also shows the end block assembly 400 according to a further aspect of the present invention . the end block assembly 400 comprises an anchor block 403 , which is fixed to the fixed structure ( not shown ). any suitable anchor block 403 will suffice for the present invention so long as it is capable of carrying the loads applied by the cable ends and also accommodating trimming cylinders 405 a , b which connect the cable ends to the anchor block 403 . the trimming cylinders 405 a , b may be hydraulically , pneumatically or electrically operated , so long as they are capable of firstly , holding the cable ends in a fixed position resisting the applied loads and when in use , being able to adjust the position of the cable end for any of the activated trimming cylinders 405 a , b . to demonstrate the effect of the trimming cylinders , an example is shown in fig3 , whereby a trimming cylinder 405 b is associated with a second carrier assembly 110 b . extending the ram of the trimming cylinder 405 b has the effect of extending the length of relevant cable 10 b , d . this has a consequential effect on the position of the second spreader assembly 120 b by lowering 410 the load carried by the second spreader assembly 120 b . thus , whilst the single hoist 100 operates to maintain the loads in the same position relative to each other , the trimming cylinders 405 a , b of the end block assembly 400 operate to make incremental adjustments of the relative position of the loads . thus , if two freight devices , such as shipping containers , are to be lifted from a ship that may be listing , or from a truck positioned on a slope , then the containers may be engaged and lifted simultaneously by “ trimming ” the position of each of the spreaders to accommodate the different positions of the containers . on engaging and lifting the containers , the relative position may be adjusted again by the trimming cylinders 405 a , b to match the conditions of the ground , ship or truck to which the containers will be moved . fig4 shows a duel spreader assembly 300 according to a further aspect of the present invention . whilst the present invention is useful for single spreader hoist systems , many of the inventive features are particularly useful where a duel spreader is incorporated . to this end , the duel spreader assembly 300 according to one aspect of the present invention is provided to be suspended from a trolley carrier unit for instance the split trolley 200 according to another aspect of the present invention . the duel spreader assembly 300 comprises an upper head frame 305 a , b mounted to cables 307 projecting from the trolley carrier above . the cables engage with the head frames 305 a , b through pulleys 308 which permit the cables 307 to pass continuously through . the head frames 305 a , b are respectively mounted to spreader frames 310 a , b . the spreader frames 310 a , b are modified in comparison with existing spreader frames in that they include a conventional attachment , such as twist lock , at peripheral corners 330 and , at internal corners 335 of the spreader frames . thus , the spreader frames 310 a , b may act individually so that each is attached to a single freight device or alternatively , the spreader frames may be drawn together and connected so as to act as a single spreader frame engaging a single freight device . the adapted spreader frames 310 a , b are mounted to the head frames 305 a , b through shear pins 320 a to d . whilst it is possible to mount connection , such as twist blocks , to the header frames 305 a , b to engage with the spreader frames 310 a , b , the nature of this connection is such that a rapid engagement is not required and can conveniently be done on the ground prior to use . thus , the use of shear pins 320 a to d provide a convenient and simple means to mount the spreader frames to the head frames . typically , shear pins of 100 mm diameter can be used , but this will be subject to the loads for which the apparatus is rated , which will be clear to the skilled addressee in the circumstances . a challenge for duel spreader frames of the prior art is how to cope with clearance of adjacent containers held by the duel spreader frames . in the case of the present invention , there are provided in this embodiment , hydraulic actuators 315 a , b , c located on the head frames 305 a , b . the actuators 315 a , b , c are arranged to have two outer actuators 315 a , c acting perpendicular to the major axis of the spreader units and a third inner actuator 315 b acting within the same plane as the outer actuators 315 a , c , but inclined to the spreader frame main axis . this particular combination of actuators permits a variable range of orientations of the spreader frames to accommodate the different positions of containers to be lifted or lowered . it is recognized that it will not be in every case that containers will be located on flat ground and perfectly square to each other . in situations where the containers are offset or at inclined angles to each other , that the duel spreader assembly 300 according to the present invention , will still be capable of engaging and lifting said containers . for instance where the containers are offset from one another by activating the internal actuator 315 b , the spreader frames will move in opposed directions , co - linear with their major axis , that is one forward and one back . alternatively , if the major axes of the containers are inclined to each other , so too must the spreader frames be oriented . in this case , the internal actuator 315 b will be free to move and the first and second outer actuators 315 a , c differentially activated so as to rotate the spreader frames relative to each other until the desired inclined orientation is achieved . these movements are based upon the spreader frames remaining in the same plane . if , however , the spreader frames need to be outside of a common plane , this may be accommodated according to this embodiment of the present invention . circumstances where it may be necessary to use the spreader frames in different planes include when containers of different heights must be lifted or where the containers are on a slope , such as for a truck on an incline , or from a ship deck listing to one side . in this case , the portions of the head frames 305 a , b to which the actuators 315 a , b , c are attached , are in fact rotatable 325 a , b . thus , by releasing a brake on the rotatability of these portions 325 a , b , the spreader frames can be hoisted at different heights with the actuators acting to maintain and adjust these height differences . it should be noted that in a further aspect of the present invention , the trimming cylinders of the end block assembly 400 may be used to adjust the height of the spreader frames 310 a , b so further adding to the flexibility of this embodiment of the present invention . fig5 shows a split trolley 200 according to one aspect of the present invention . whereas conventional hoisting systems will have a unitary carrier acting as the trolley from which the spreader assembly will be suspended , the present invention in this embodiment provides for two discreet carriers 205 a , b linked by actuators 210 a , b . the split trolley according to this embodiment is particularly suited for use with a duel spreader arrangement . in such an arrangement whereby spreader frames will separate so as to accommodate side by side containers , trolley carriers according to the prior art are in fixed relation . therefore as the spreader frames separate , the cables from which the spreader frames suspend will become more and more inclined , leading to greater tension in the cables to accommodate this angle . in the present invention , the actuators 210 a , b are activated so to separate the discreet carriers 205 a , b to a position corresponding to the separation of the duel spreader frames such that the cables become substantially vertical . whilst the trolley units 205 a , b are discreet units , said carriers are nevertheless linked through the actuators 210 a , b . thus , movement of the trolley as a whole unit need only rely upon one of said carriers 205 a to have a motor such that the trolley will run along the crane rails of the fixed structure . to this end the split trolley 200 is in fact divided into a master trolley 205 a and a slave 205 b , whereby the master trolley 205 a is motorized having a motor of sufficient capacity to also drive the slave unit . fig6 shows a further embodiment of the present invention , whereby the split trolley involves movable sheaves 355 , 360 , 365 within a single trolley carrier 350 . as with previous embodiments , the arrangement includes a trolley 350 from which is suspended , using cables 367 , 370 , 375 , a spreader assembly 380 . in the embodiment , a first position of one carrier 365 is shown whereby the corresponding cables 375 are inclined , and so increasing the applied load . the second carrier 355 has cables 367 in the desired position , being substantially vertical , and hence it is desirable to move the first carrier 365 so as to have these corresponding cables 375 also vertical . hence , the first carrier 365 is moved 385 to a position 360 , whereby the cables are now substantially vertical and thus reducing the load in the cables , and otherwise benefiting from the more desired arrangement . as a further alternative arrangement , fig7 shows an embodiment of the trimming cylinders 500 . in this arrangement , a hoist drum 510 , controls cables 505 a to d . the cables pass through sheaves before progressing to a trolley and head frame ( not shown ). the intent of trimming cylinders is to vary the length of cables either individually or collectively . in the previous embodiment , this was achieved by extending an in - line cylinder . in this embodiment , the lengthening comes from decreasing the path through which the cable passes . specifically , the trimming cylinders 520 a to d bear directly on the sheaves 515 a to d , and in fact , do not contact the cable . the sheaves 515 a to d are movable subject to the force applied by the cylinders 520 a to d . thus , by retracting the cylinder , the sheaves 515 a to d move 525 a to d accordingly . by moving inwards , the path followed by the cables 505 a to d is shortened , leading to the corresponding portion of the head frame lowering relative to the non - affected cables . conversely , by extending the cylinder , the sheaves are moved in the other direction , lengthening the path , leading to the corresponding portion of the head frame lifting relative to the remaining portions .