How can I wire an exhaust fan to turn on from multiple locations?
I don't think it's possible to individually control two different lights while controlling a common fan using conventional house wiring devices. Powering the fan will backfeed power to the other light that was supposed to stay off. But I look forward to being proven wrong.

It can be done if both lights and the fan all go on at the same time. This is just a standard 3-way switch circuit. Alternately, wire two standard switches in parallel so one bath user would not accidentally turn off the light for the other. This can be confusing as one switch may not always turn off the lights. 

To have only one light go on with a particular switch, you need a pair of single throw-double pole switches where one pole of each switch goes to it's associated light and the other pole of each switch both go to the fan. While such a switch is common in electronics, I don't know if such a device is available for house wiring.

Alternately, a double pole relay could be used to switch the fan. Either pole powers the fan, but the separate coils keep each circuit isolated from the other. Sorry I'm not illustrating this, I need to take some time to learn how to add drawings here, I know it's fairly easy. In the mean time, if anyone gets what I'm saying, feel free to add some illustrations.