Unless the fire truck is told otherwise, it should go to the fire that it was originally sent to.  The dispatcher will have a much better picture of what resources are available to fight both fires and they should make the decisions on which truck to send to which fire.\n\nLike with any emergency service, the situation can change from minute to minute.  The fire truck should radio the dispatcher and inform them of the new fire.  The dispatcher might already have another fire truck on the way or they might redirect the truck to the recently observed fire.\n\nYou also have to consider the type of fire involved.  A fire at a hospital full of patients will have higher priority than a fire an an empty warehouse.  Each case is different.