There's probably a whole thread on this single topic (@magpie patrick - you listening?)
Logically, separate windlass's should not be the norm - if someone asked you, without prior knowledge, to design a sluice, would you not have a fixed handle or crank to operate it (I've never seen, for example, a bucket in a well with a removable handle)
I assume (but that's all it is) that at some point someone said, "what if someone unqualified came and operated the sluice?" and the obvious answer in a pre-stilsons era was to remove the handle and give "qualified people" their own handle to bring with them.