If a swarm of GS move to a position that already has a swarm of GS (that haven't yet moved), does the currently moving swarm join to this second swarm immediately or do you first check that the second swarm can move (and only add the currently moving swarm to this one if it doesn't move)?

Cheers

Rulebook, page 22: Regardless of how they move, each genestealer card may either move or flank, at maximum, once during each Event Phase.

So, it doesn't matter if the second swarm is going to move afterwards or not, because the first swarm will stay there no matter what and only merge definitively if the second swarm doesn't move.