Fastsky:

Yaw stability is effected by the length of the wing vs the length of the fuselage. If you extend the wing beyond the designed extension, (which is one bay on each side), you will need to extend the fuselage to compensate, or tracking and engine-out stability will suffer. You could increase the size of the tail, instead of extending the fuselage, but that is what the designer did in order to compensate for the designed extension and making it ever bigger might make it look a bit odd.

My first Super Sportster Twin flew just like the single engine Super Sportsters. Of course everything was built heavy in those days, so building it as light as possible can only help. I am building a Super Sportster Twin based on the SS 90/120 right now, and my wing extension is just slightly less than one bay per side. I am using light balsa for the blocks, tapering the hardwood wing spars and leaving out most of the plywood in the fuselage to lighten it.

Jim