Why, if you do have enough rooms for each kid to have their own, are you wanting to put 2 of the kids in the same room?
Since you do not HAVE, to?
If you don't have to, I wouldn't.
Leave things as they are.
All I know is, when I was a kid, I shared a room with my sibling who is only 2 years older.
I HATED HATED HATED HATED HATED, it.
We are of two different personalities, and had 2 entirely different sleep habits and routines, and it was just oil and water. And I just could not stand it.
So FINALLY, my parents put us in our own, separate, rooms.
And then, it was much more peaceful and I got sleep...AND it was also the only time, I did not have to be around my sibling. I got my own "me" time, when I was in my own room and able to do what I wanted and my own space to do so.
Doing a little remodeling, of the last room, to me, just has to be done no matter what. If not now...then you'd have to do it later, anyway.
So just do it now.
And most times, when someone thinks that "later" it will get done... well, through procrastination, it may not get done.
The "idea" of the kids sharing a room, may sound idyllic.
But it is not always.
Unless you HAVE to, combine them in a room, I would not.