As far as I know that isn't a problem.
You do have the full version of FSUIPC, don't you?
I've been telling people to get it for almost as long as I've been using FSX.I use it with multiple switches to start engines one at a time, and a ton of other stuff.

Among other things, you can use it to send multiple commands to FSX too.
So with 4 switches, normally set one for engine, I could reassign 2 of those switches if I were flying a 2 engine aircraft for some other purpose unique to that aircraft.
Once set up in FSUIPC, all
you have to do, is to remember your switch assignments after selecting your plane.