Make it simple and pretend there's no wiring or switch for your lockers at all. Install your own switches and wiring, it's as easy as wiring two switches like you're controlling two 12v light bulbs but they're compressors instead. No need for relays at all, those little compressors don't draw enough current for that. They also shut themselves off automatically when there's the required 5 psi of pressure in the system.
I installed these two switches for my Rubicon locker compressors, they are 100% independent of each other so you can turn them on in any order you want. Each switch/compressor is fused with a 5 amp fuse.
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