2 Different Circuits in 2-Gang Switch Box

I am adding an exhaust fan in my bathroom and moved from a single gang to a dual gang box where the switch that controls the overhead light is at. The existing wiring for the light switch is actually wired as a switch loop, so no neutral, so can't pigtail and use that circuit for my fan.

I can easily get another circuit in the dual gang box, but was curious if anyone knows what code says (if anything) about this situation. I can't use 20a double with common neutral so that one circuit trip trips the other since there is no neutral due to the switch loop. Should I just write in the metal box and on the back of the wall place that "This box is served by 2 separate circuits"? I am not sure this is code required, but maybe courtesy. Although, if someone works in a box and doesn't test all wires for hot before working, there really isn't anything I can do about that.