Okay, here is my advice to anyone dreading making this transition.
1. Just do it.
There will never be a time when you feel ready enough to make this transition, because babies are babies and once you feel like you are on a schedule, they get a tooth or go through a growth spurt or something happens that makes it feel like you should wait. No matter when you decide to do it, there will be a transition period so no need to put it off. Just rip off the bandaid.
2. Play in the room with both kids
In order to get Titus used to the fact that it was his and Cecilia's room, we spent a lot of time sitting on the floor in his room and playing instead of playing in our living room where there is more space to move around. We read lots of books together, put Cecilia in her crib and talked about how it was Cecilia's crib where she would be sleeping. We practiced whispering when she was "sleeping" in her crib so he would learn to be respectful of her sleep.
3. Be prepared to go with the flow for a few days
Because they may be tired and cranky the first few days from restless sleep and nap-fighting, your days will probably be thrown off for 2-3 days. I took them to the gym with me which was a good distraction for them and it helped break up their day, but nothing was accomplished in the afternoons because they didn't sleep during their normal nap time and then we just had to drive so they could get that rest. We also watched a lot of Thomas and Friends. A lot. I don't know how much more I can handle of those Useful Engines and their Troublesome Tracks on the Island of Sodor. That show is so annoying, but it chills Titus out like a pro.
4. If at first you don't succeed...stay with it
It's no secret that change is hard, especially for kiddos, but like with any other life change, they will adjust. Some kids may take longer than others. Titus wasn't a fan for the first couple nights, but after a while, they began to soothe each other and find comfort in their presence. I encourage you to not let those first few nights discourage you. It's so worth it once they adjust. Listening to their sweet voices in the morning chattering nonsense back and forth across the room is the most beautiful sound you will ever hear.
Friends, I wish you luck with your transition.