I might be wrong, but I beleive each seperate bathroom must be on its
own 20A circuit.

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I thought I had read or heard somewhere where some referenced the all-
mysterious "code" and said that if there was nothing else on the
circuit except the outlets, then it could be used for another
bathroom's outlet. If I put the master bath outlet, fan/light, vanity
light, etc. on that 20A circuit, I think I could not jump to the other
bathroom.

Can someone confirm?

BTW - the NEC really needs to make it easier for DIYers - I don't know
- maybe a searchable database, FAQ, knowledgebase? I know it keeps
electricians in business b/c then no one bothers & they just hire an
electrician who understands the almighty "code".