I realize it may have been answered but there are so many nuances I want to be sure: I have one (metal) double-gang box with one outdoor light circuit. I want to add a switch for a completely different outdoor light. The two independent circuits would co-exist in the same box. Is this legal? Should I put a warning note in the box there are two power sources (on different breakers) in the box? The obvious danger is someone switching one breaker and thinking the box is safe.
I mix multiple circuits in the same box a lot.
I would color the wires for the second circuit differently.
Use gray shrinkwrap or tape on the second circuit's neutrals. Then use brown or blue shrinkwrap or tape on the second circuit's hots.
If the next guy is dumb enough to mix gray neutrals with white neutrals, well, you can do what you can do, but you can't fix stupid.
I for one prefer to also use a differnt color for switched-hot (typically red), so mark the switched-hot for circuit 2 with both brown and red.
You can have multiple in the same box if they are the same gauge wire.
You are probably running 14/2 for your lighting. If you want to be extra safe about the box then you use a breaker tie in the panel to tie the two breakers for the two circuits together such that if someone flicks one off to work on it, both are flicked off. This used to be done for kitchen outlet devices - before code changed to 20amp outlets - where each outlet in the device is on a separate circuit.