Please, please, please don't do this. I was performing a prebuy on an RV-8 today and for the second time I found a fuel valve with a BOTH position. Huge NO-NO! A both position in a low wing aircraft will fool you into thinking it works correctly, especially when both tanks are either full or above the fuel pickup. But let one of the tanks get below a fuel pickup and even if you happen to have a full tank on the other side, it will suck air. Probably at the worst time, like when turning base to final. More accidents are caused by modifications to the fuel systems than any other modification. It was a even very nice Andair valve, too.