The needs of the service come first, family a close second (according to the MCM Div mission statement).
Both me and my wife are serving, we have never been posted to the same unit but MCM Div have always tried to put us both in the same geographical area. Except for when I was in Camberley and she was in Portsmouth, our quarter was in Pompey and the commute every day was a complete pain in the backside.
The RSPOD paid for the petrol but the cost of frequent changing of tyres and the car servicing which came round quicker than the norm came out of my pocket.
As for operational tours, we have never been deployed to the same theatre together, but we have been deployed on different ops at the same time (which wasnt too bad especially if you can manage R&R at the same time) and on 2 occassions her unit replaced mine (which was a nightmare as youll see each other for 2 weeks R&R in each of the 6 months).
Its a great life, especially when you move up the ranks and into the messes.