We do try to keep boarding sows and boars separate, in different rooms if we possibly can, but sometimes that's just not possible.
If it's not possible to house them in different rooms, then we're super careful about smells......firstly, we try to boars and sows on opposite sides of the room, and if that's not possible, then (as Wiebke said) we always have boys above girls so that no poops or soiled bedding drops into the boars cage at clean out time.
All of our cages have solid wood dividers between them, so there's no visual or physical contact between boarders. We have different cleaning tools for boars' cages and sows' cages, and we sterilise the tools daily. We also use a number of disinfectants/deodorisers when cleaning out the cages and steam clean them between residents - just to be on the safe side, LOL!
So far, we've only had one pair of boars fall out whilst staying with us (and, believe me, we've had dozens and dozens of boar pairs staying with us). We assumed that their fall out was due to being in proximity to sows, but the owner confessed that they were on the verge of falling out before they came to stay (:{) and they'd forgotten to tell us!
So, yes, boars and sows can live in the same room, but you really do need to take super-strict precautions and keep a very close eye on the boars' behaviour towards each other.
Tracey x