I am in the process of wiring a new bathroom circuit that includes normal lighting in addition to a fan/light combo. There is a three-way switch that will control the light in the bath fan (as well as the other lights.)
Another switch controls the fan. The three-way switch (lighting) is serviced by a 12/3 run, and the outlets and fan switch are serviced by a 12/2 run, both on the same circuit (notice the single "source power".)
The light part of the fan/light combo is served by the hot on the 3-way switch. The fan is served by the hot from the "outlet" run switched by the fan switch. The neutral for both is from the "lighting" run. Notice the neutral from the "3-way run" connected to the "light" and on to the light/fan.
Boiled down: is it allowed to use a single neutral on the 12/3 for two "hots" that are served from separate runs of the same circuit? Possibly similar to this answer.
Picture of the circuit: