Bathroom exhaust fan and light switch separation
It looks like you have a switch leg but they should have used x3 with ground cable. The black mark on the white cable I would guess is your hot the black is the switched hot that is working. I normally would have the 2nd switch on the red wire with the 14-3 or 12-3 wire from the fixture. with the white marked black always hot jumped to both switches With the extra cable this may violate code depending on how it is wired. We would need a photo of the wiring connections at the fixture or box where this is fed from. If the 2nd cable is connected as a switch leg white always hot then the black to the fan  it would be ok. On an older home that did not require a neutral in the box.