"Any reason that would not work (other then they both need to know both jobs)?" Yeah, mainly that. And switching between two jobs means loss of focus. And if one crew is out--now you need a replacement with dual skills. So it is possible, it just isn't always an optimum way to go.
"the clutch on the ST
winch just slows him down." A lot of folks don't throw the final bit of
line up into the clutch until after they've hauled in all the slack. Then they take up the last turn, slap in the handle, and bring in the rest. That is, assuming they need the handle. Sometimes by tacking a bit slower, you give the crew time to haul in tighter--by hand--and there's less need for the
winch handle and the final trim.
If the off or extra crew can be the one to slap in the handle FOR the job trimmer, that also speeds up the operation.