The NEC at article 210.11(C)(3) states-
Bathroom Branch Circuits. In addition to the number of branch circuits required by other parts of this section, at least one 20-ampere branch circuit shall be provided to supply bathroom receptacle outlet(s). Such circuits shall have no other outlets.
Exception: Where the 20-ampere circuit supplies a single bathroom, outlets for other equipment within the same bathroom shall be permitted to be supplied in accordance with 210.23(A)(1) and (A)(2).
In article 100 - Definitions
Receptacle - A receptacle is a contact device installed at the outlet for the connection of an attachment plug. A single receptacle is a single contact device with no other contact device on the same yoke. A multiple receptacle is two or more contact devices on the same yoke.
Lighting outlet - An outlet intended for the direct connection of a lampholder or luminaire.
Lighting outlets, other than lamps do not have attachment plugs. So they do not qualify as receptacle outlets.
Therefore if the circuit leaves the bathroom to go to another bathroom you cannot put the bath lights on the circuit.
Just to clarifly that to put light outlets on the circuit it cannot leave the single bathroom.