Another update, but still not solved, just some new observations.
Boat sat at the dock for a few days while the wind blew crazy. First day after, had a major engine stall issue, probably 10-12 times right after we got started and were putting around - affected alternating engines. Next day on the putt out, only 1 stall. Next 2 trips, not a single problem. Last trip, after running ~4hrs and trolling, one engine stalled.
I've also noticed that the stalls are often accompanied by the RPMs on the engine about to stall revving higher, then rapidly dropping.
It's those days of no-stalling after a lot of use that is making me think this is not gas supply related....