The Breaker Finder 5000

YY1 said
I've been bitten by a neutral more than a few times.

I'm in the long process of installing 20A receps in my shop, arranged as one from each phase in a two gang box.
They will be spaced either one or two per 20 foot bay along the back wall.
I'm using white receps for the "black" phase and gray receps for the "red" phase.
I'm thinking about using yellow wire for the neutral of the red phase, with white tape.
If anyone knows the proper way- so I can differentiate at the recep.....
You can't "just" use the same neutral, or they're not truly "separate circuits".

...and I have a portable 240V compressor....and have considered making a 20A 240V female plug to two male 120V plugs, so I can "simply" plug that into one each of those separate phase circuits and have 240 anywhere along that 80 foot wall

Avoid being bitten by the neutral by using a 2 pole breaker. This way the neutral won’t be able to bite you.

For your application I’d recommend sharing the neutral, it’s less complicated.

A “Tie-Bar” will de-energize both of your 120v circuits that you plan to use temporarily as a 240v circuit.