Hello all, i'm a tile contractor looking at a custom walk in shower, the size of the shower pan is approx. 8.5 ft wide by 3.5 deep, with a showerhead on each side of the short walls

local plumber tells me you need (2) 2" pvc drains for this setup, HOWEVER because this is a handicapped accessible shower the customer has already lowered the subfloor under the shower area so that the finished mortar bed height is equal to the rest of the floor.....no curb, and wheelchair rolls right in

the lowered floor means some sistered joists, and this takes up the needed room for 2 drains.....sooo the plumber says we need (1) 3" drain instead

1) is this correct, i live in the st. louis mo area if that makes any difference
2) my real problem is that as the tile contractor i want to use a shower waterproofing system made by Schluter Systems called "KERDI" and they make their own drain to fit their waterproofing membrane, and it only comes in 1 size of course, the standard 2 inch....i don't know if there is a way to modify a standard 3" drain but maybe some of y'all might know about that??