introducing the second member of the first pair and the first member of {HEAD ~ the second pair} into a second vessel from {TAIL ~ the first vessel} 
introducing the second member of the first pair and the first member of { HEAD ~ the second pair } into a second vessel from { TAIL ~ the first vessel }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL