Maths!
I play in 3 bands and sub with a 4th.
My #1 band gigs regularly, usually nights, and takes precedent (and importantly makes the best money).
My #2 band rarely gigs (so far only afternoons at festivals) but does a weekly live stream to Facebook on Monday nights so #1 and #3 know not to schedule rehearsals then.
My #3 band also does mainly afternoon festival gigs. The BL is the guitarist from #1 so there is never any conflict there.
I sub when required with #4 if not busy with #1, #2, or #3.
Occasionally I get an afternoon gig with #2 or #3 and a night gig with #1, no big deal.