I have to say I'm with petal
We used to live in a 2 bed house as that's all we could afford, dsd and dss had to share a room they were 14 and 9 when this started and it continued till last year when they were 16 and 11, we made the best of it, dss has a high sleeper bed and dsd's bed went under it then she had a curtain round it for privacy, luckily it was a large room so their was space to divide it with a curtain down the middle with their drawers, toys etc on either side.
Last year we were fortunate enough to be able to buy a 3 bedroom house but I'm expecting a baby in march, the 3rd bedroom is going to be the baby's room and dsc continue to share. The way both I and DP see it is that they are only here 11 weeks of the year (they come for school holidays due to how far away they live it's not possible to have them for weekends sadly), they have a room each at their mum's and most of their stuff is there, they still have a bed, chest of drawers, toy/art storage each but they have to share a room. DS when he arrives will only have 1 room in which to store all his stuff and will be a permanent member of the household and therefore takes priority over those not here all the time. If one of the dsc decided to move in with us then the 2nd bedroom will become theirs as they will then become a permanent member of the household and the visiting child will have a blow up bed/camp bed/sofa to sleep on whilst here. If both decide to move in then obviously we will shuffle rooms so that the boys share and dsd will get the small room.
As a compromise to dsc sharing, the conservatory has been turned into a sort of den for them, there's a TV, playstation, sofa etc out their so the bedroom is literally the place they sleep in, there is an additional living space which is where dss spends most of the day and dsd tends to stay in the bedroom then in the evening they swap so when dss goes to bed dsd goes into the conservatory to watch TV and Facebook her friends. Is this possible OP? Do you have a space which could be turned into a den so dss is literally just using the bedroom as a place to sleep?
We discussed the bedroom situation and our reasons with dsc and they are more than happy with the way things are, if either of them seemed unhappy we would reconsider but for us it works and they are certainly not 2nd class members of the family they are a very important part of our family they just aren't here all the time.