One strap or two? On a melodeon, I find one is better if you're sitting down. You may need two if you have to play standing up. With one strap (seated) you can comfortably stretch the bellows away from you at any angle that suits, whereas with two, you have to stretch at a particular sideways angle and I find it's not so easy to push and pull this way.
Accordion players usually need two straps, as the instruments tend to be quite a bit heavier, and the action is different - they don't need to push and pull so frequently, it's more of a steady heaving in and out.
Just to make it clear what I mean by these terms:
Melodeon = diatonic button box, the standard one having 2 treble rows and 8 basses.
Accordion = big brute with piano keys and usually dozens of basses. Not diatonic (can't think of the word - monotonic? monotonous? :-))

Marje