Originally Posted by
zzyzx_xyzzy
Sure, the seat/crank/bar relationship should be the same as on single bikes, but that leaves the question of how much space to give between the stoker's bars and the captain's seat.
That would depend on where you mount the stokers stem and how long the stem is. The stem length will determine the length of the stokers area beyond the handlebar/seat location as they should be fixed by the fit geometry. If you are making a custom stem then you have some leeway to make it a little more roomy.