I currently have two bathrooms serviced by one circuit. Each bathroom has lights, a single GFCI counter-top receptacle, and one bathroom has in-floor resistance heating. I will be adding in-floor heating to the other bathroom. As I understand it, code requires a two circuits for two bathrooms, but allows two configurations: 1) all lights on one circuit (but not shared with any lights outside the bathrooms. and the other circuit serving the outlets for both. 2) one circuit for each bathroom.
I prefer to keep lights on a separate circuit from the receptacles, since 14 gage wire is easier to run and I am forced to do some some long, difficult runs (vaulted ceilings, solid log home), Also If a breaker trips, it's nice if the lights stay on. But I also think putting both bath receptacles on the same circuit increases the chance of nuisance tripping (two hair dryers at the same time > 20A?). I'd like to put the bathroom lights on a bedroom light circuit and give each bath receptacle it's own circuit, but this appears to violate NEC.
I'm curious what the rationale for these rules are. I'm also interested in the pros and cons of each configuration. Also would like some input on whether or not the heaters should go on the receptacle or light circuit for config #1. I would prefer putting them on a dedicated circuit, but unfortunately that is not an option. The heater load fits easily in the 20A available.