Hi N., do you possibly have a pack N play? I use one here for my 18 m/o gr son during the day, his older brother lays on a reg bed at age 4. If a pack N play will fit in the little girls room with still room to play etc, or in your room until you move him, that might be an option, when the night time feedings are over.
I just use extra quilts in the bottom for more padding. All the Gr kids used it when they were babies staying here.
I think the sooner you can move them is ok, they get used to each other being there quicker. We kept our youngest with us until he was 6 months old, then moved him in with his brother. Didn't seem to be a problem with either one of them. We didn't have the Plus of baby monitors either.
God Bless and hope you got other great ideas or advice.
I didn't get to MS until now, but we have garden things bought...lol Now stop Raining
K. Nana of 5