Bit of a can of worms.
Chez are you saying that your couple share a single room and occupy that as their home, and don’t occupy the other one.
Looks to me as though you should question the L/L’s motive here, either they occupy both rooms or they don’t.
If the L/L is turning a blind eye to the fact they aren’t single in order to continue providing them with Accom for single people, why should the benefits pot pick up the tab. Why doesn’t the L/L simply bite the bullet and put someone in the other room?
Alternatively if they are occupying both rooms there is no doubt in my mind that you could treat that as one dwelling and aggregate the rents. I’d want some assurance that they were occupying and using both rooms and that this wasn’t just a way of getting around the first problem. If one room was for sleeping and the other for sitting and watching TV (or whatever else it is they might do in their room(s)) I really don’t see how this would be anything other than one dwelling.
Looks like its down to the L/L really.