so, if I've got a Bill Ferny (first set) and a Witch-King out there, I can't let the freep player assign "the king" and then assign Ferny in the same skirmish, can I?
Well no, but that's for 2 different reasons, 1) because unless a companion has defender bonuses, it can only be assigned to 1 minion by the FP player and 2) because Bill Ferny forbids the Free Peoples player from assigning him to a skirmish in the first place.
However, in your example, FP player assigns Witch-King to say, Aragorn, then you get to put Bill wherever you like, even on Aragorn.
Thranduil