hi jerry im asuming your a spark are both bathroom lights on the same circuit ,if so you could use two small n/o contactors  then using a three core cable from both roses connect the switch wires from both bathrooms  to the coil side of each contactor then the neutrals to the other side of the coil, then a perm feed to the contact in side of the contactors  and then contact side out of both contactors to the fan switch connection then a perm and a neutral to the fan  

Thanks guys, very helpful.  I think the way it has been done in the other buildings is the double pole switching suggested by plugwash.

I don't know why I didn't think of that!!

Looking at your sketch (thanks for taking the time to do this), I assume I could link the hard live across the switch poles rather than take two live feeds from the ceiling rose.

Anyway, many thanks, you've solved my problem, I've spent ages trying to work this out.