Coming from the perspective of the oldest child here, I still think DD2 should get the better room. When we moved house I got the biggest room on the understanding that when I went to uni, one of my sisters could have it. When it came down to it, neither of them could actually be bothered to move, but they went in there if they ever needed more space, etc. I just thought I was lucky to have any room to come back to in the holidays, and somewhere to store my stuff. Lots of people don't!
When DD1 is saying where is her stuff supposed to go, well, where is DD2s stuff supposed to go now? Or, because she doesn't have anywhere to put it, does she have fewer possessions than DD1 as well as less space?
Give DD1 the choice: either she can keep the room as hers to come back to, but DD2 can use it whenever she is away, so her friends can sleepover, possibility of them going through her stuff, sleeping in her bed, etc.
Or she can have a room, that is totally hers and no one can go into, she can redecorate herself, etc, but it will be smaller.