2,2 yo and 1 yo to start sharing a room- what's the best approach?

[notsuchayummymummy]

2.2 yo has been in her own room since 6 months old. 1 year old still in with us and not yet sleeping through.
We really think that the time has come to move them in together- but are we being unfair to our eldest when the youngest still often wakes for 1 night feed.
Any thoughts / guidance / experience appreciated- I'm bricking it!

Ds and dd started sharing when dd was 6months and ds was 2y2m. He slept really well and never woke when she did, which was at least 2 times a night.
The only time we have had to move ds was when I spent time I hospital and dd had to go cold turkey with bf. dd had to be put down crying as dh didn't have anothe option ( she would not take a bottle and he had to deal with ds) Ds would then have a story in our bed and go in once she was asleep.

Now they have a story and then lights out and sleep. Ds waking will wake dd, but dd waking doesn't wake ds