You can't really put two AC supplies in parallel because if they get
out of phase they'll start eating each other's power as fast as they
possibly can and will blow up

Are you sure you're right, here? After all, the National Grid is supplied by
no end of alternators all in parallel. Once an alternator has been put on
line it remains in phase automatically because it can't do anything else.
It's getting it on in the first place that's the trick (but not a big one).
Lamps across each pair of phases will go out when the phases are in phase,
so to speak, and the alternators can then be switched together.

Rob Graham