The big issue with twin main foils as I see it is induced drag at takeoff. You really want maximum span there. With two foils you get two more tip vortices than needed, and much more drag when you can least afford it.
The strut drag is significant, but with two struts the main foil can be so much higher aspect that it outweighs the loss from the struts, and the struts can be much smaller also. I suspect we may see twin strut moths at some point with the Clive system if the fairing drag can be worked out.