Good explanation.

Faced with that, ie. the wife on the flute and me on the trumpet with Band in a Box you need to print the music in the actual key (as John said) so if in Band in a Box it's written in C and sounds C you do that. Your options are to print both part, or have one person do the other part from band in a box.

If you find the P for Practice icon, there's a button to transpose the piece for Bb trumpet. Press that and note that the music goes to the key of G if you were in F and the you can play the trumpet along with it. Note that you are shown the piece on the screen in G but that it's still playing in F.

Probably some way to deal with the guitar and a capo but I'm no guitarist, however if you can stick one on and both end up in the same key that's another workaround.

Playing the trumpet and the guitar at the same time must be a hard thing to do.

John Conley
Musica est vita