It looks like the bolt circle on your crank is 110 mm so you can go as small as 34T for the inner drive chainring. The only problem I can see is if the inner drive and synchronizing chainrings are the same size or the inner drive ring is smaller, the two chains might not clear each other.
I've never seen a tandem set up like this before. Usually the synchronizing rings are on the other side of the bike. I guess this arrangement allowed them to use two conventional (and cheaper) cranks.