The Breaker Finder 5000

That's what I was originally going to do, and that's how it's currently wired.

But I read something that said if you wanted to work on one circuit, you'd have to turn both breakers off.

Yeah, all the neutrals are eventually tied together in the panel.

(and don't come into play at all in 240)

It's been a bit since I've worked on or thought about this project.

I got 2 boxes wired up and that has been enough so far.
(WAY better than the single, shared 15A recep on the other side of the shop)

Maybe that thing I read said you couldn't draw 20A from each circuit.....

Then what you said about up-rating the neutral makes sense.
I've never heard anyone ever say that before, usually it's "de-rating" the neutral.