We had to put our 7 and 4 year olds in a room together for about 7 months. Our 7 yr old was a boy and our 4 yr old the girl, though. It was never easy getting them to sleep. It was very stressful for me, and sometimes them, but mostly just me. They seemed to be having a GREAT time.
There were occasional squabbles over things, but the going to sleep issues were all b/c they didn't want to settle down and go to sleep, or they didn't want the same song/music on the CD/radio. We were building a new home, and no rental (except the 2 BR townhouse we ended up in) would take us due to our German Shepherd family member. :( Once we got moved into our new home, they went right back into their own separate rooms again, and bedtime got much easier for me again. They STILL like to share space at bedtime much of the time, though. On weekends, they will ask to have a "sleep over night". Or if we have a guest that needs one of their rooms, they will want to sleep on the other ones' floor in a sleeping bag. It is great fun for them still, and they are now 12 and 9. But those nights in a single room were very hard for me. I basically had to "stand guard" outside the room on the staircase for 20 minutes until they drifted off to sleep. Oy.
But if you are doing this long term, they may grow into a more calm routine. Mine knew from the get go that it was temporary. I don't know if that mattered or not. I DO know that they look back very fondly on those days they shared a room. I never had sleep issues with either of them. No nightmare problems, or anything like that. So, if it is less work to get them to sleep than it is to deal with a stomachache every night or nightmares in the middle of the night... then I say give it a shot. Eventually they will want their own space again. So keep that in mind. It's fun while it's temporary... when it isn't their choice anymore, squabbles will be more frequent.