I'm not sure if I've mentioned that we moved Matthew into Elizabeth's room when we got back from the cottage, but the two kids have been sleeping downstairs in the bunk bed - Elizabeth on top, Matthew on the bottom. Mostly it has worked out. Elizabeth's been less possessive about "her" space and "her" things (because she's had to share it!) and Matthew's been less of a nuisance with said things (I think because it's not a forbidden room anymore).
There have definitely been some downsides, with Matthew sometimes waking up yelling in the middle of the night and in turn waking his sister, who responds by yelling back "Matthew is keeping me awaaaake!! STOP YELLING!!" This helps about as much as you might expect.
Tonight they decided they would have a sleepover in Matthew's bunk. It was adorable. Of course Matthew and Elizabeth both kick like crazy in their sleep so it didn't last all night but Elizabeth didn't have far to go!