You can create a plane that looks like a biplane (i.e. so that the 3D model of it is a biplane), but there is no way to tell the simulation engine that there are multiple wings and what their location is etc. Quite possibly one could achieve an end result that would be good enough anyway to fool most customers of such a plane. After all, the H135 helicopter is very popular and its users and developer get a bit offended if one dares suggest that the simulator doesn’t actually support helicopters.