You need to consider everything that the other posters mentioned on this board, as you have several options.
I had the same situation with the bathrooms near each other. They were on the second floor near the attic. My criteria were:
1) Remove the moisture from the bathrooms.
2) I hate fan noise! If it is too noisy then people won't use it.
3) I wanted to minimize the number and location of the exterior holes.
4) Vents in bathrooms should be small.
I used one fan to vent both bathrooms, and I put the fan in the attic away from the bathroom vents, therefore minimizing the noise. An aded benefit is that the vent openings in the bathroom can be very small. You can use 4 to 6 inch vents rather than the huge vent units with built-in fan and lights. Fantech makes the fan and vent units. See below:
I bought the Fantech supplies from:
I put the vent exhaust hole in the sidewall of the house rather than through the roof. From ouside of the house it looks like a dryer vent from the attic (I could do this because I have gable ends).
The fan is wired so that turning on a timer in either bathroom turns on the fan. I hate the rotary timer switches, so I used neat new electronic pushbutton timer switches from www.hvacquick.com. I think they are made by Leviton. Because there is only one fan, turning on a switch in either bathroom means that both baths will be vented simultaneously. It's not a problem, as you will be using the vent for maybe twenty minutes twice per day. You can get motorized switched dampers and controls for each bathroom, but it's not worth the additional cost. The vents in the bathrooms have dampers to prevent backflow.
No matter which system you use when venting a bathroom through the attic, you need to insulate all of the vent ducts and fans. Otherwise the warm moist air from the bathrooms will condense in the cold attic and you will get water in your ducts, fans, and maybe dripping on your head!