This got raised back in 2018 when the original Squadrons implementation went in, too.
I expect it'll come up in 2032 for the next go round as well, but just in case: it's probably too complicated to allow people to actually be in multiple squadrons at once, but one thing I'd seen done back in 2018 in some other games to let people manage multiple affiliations is to let group leaders have an "auto accept application" list for their group, and I still think it makes sense to have for Elite Dangerous.
- So player A is, perfectly normally, part of wider player groups X, Y and Z.
- The squadrons attached to those groups all have player A on their "auto accept" list.
- Player A can leave squadron X, apply to squadron Y (which auto accepts them), and fly for this session as part of squadron Y. Then after a few hours, they can switch back to squadron X. Nothing they can't already do, of course, it's just simpler because the squadron membership ranks don't need to be awake.
- If player A applies to squadron W, then they're not on the "auto accept" list there and have to be approved by the squadron staff; if they then leave W again they've no automatic right to return.