It isn't against code to have different lines inside one box (at least at any of the places I've lived). Easiest way is to flip the mains everytime. I wouldn't

It isn't against code to have different lines inside one box (at least at any of the places I've lived). Easiest way is to flip the mains everytime. I wouldn't trust those pencil testers though (the non-contact ones, the light ones yes).