I tried to scetch/doodle out how the center of gravity would work on a dual seat, side by side tadpole. Looks great, real stable... unless the 2 people are diffrent weights, or one person gets up. then it either falls over, or just can't be turned one direction without some manuvering around like the passanger of a sidecar racer.
I think a Delta would be more stable, or even a quad.