Wiring bath lights that have two power sources
No to Q1, switching both lights, unless you get the same power that the existing light switches to be in parallel with the fan light.  That would mean running the same gauge 12 or 14 -2 (hot, neutral and ground ) to the fan light from the overhead light.

Combining circuits risks having 240 VAC across your fixtures.  It can be done, if neutrals and ungrounded (hots) are kept separate (from each other and not connected through a device section to another.   

Maybe to question 2, if you mean using the one (original power source) to switch fan, light, fanlight together.  (this assumes the added device wattage doesn't exceed the wire capacity.  

You would be better served by biting-the-bullet and put in a new switch box and use the separate power available and get all 3 functions separately   Am I guessing correctly that the bath outlet is on the same circuit as the light and they are 14 gauge??  One hairdryer-light-fan-fan light will likely overdraw a 15A breaker