A standard airframe will work fine for a twin. On my twins, I build for square. Same size fuse vs. wing length. If you are building the airframe, add additional vertical/ rudder surface area for engine out. Having said that, my first 2 twins were 25 size. With the engines close to the fuse, didn't make much of a difference with one engine out. I used the one finger gap between fuse and prop tip. Remember you have to get the glow driver between the head and the fuse. Otherwise,what ever wets your whistle for design.