I am the second of 4 kids, my brother being the youngest. My sisters and I shared a room thorough high school and well into college (before my older sister got married and moved on, etc.).
My older sister is 3 years older, and younger sister is 2 years younger, so there was a 5 year span in the age difference.
As much as it might make sense to keep the family harmony now by acquiesing to your older daughters request, in my mind it's going to do greater harm. Your daughters are going to have to learn to live in the same house together just like the rest of you guys do, and the sooner they figure out that out the better. I went through several years of "growing pains" with my sisters, and we all lived to joke about it. (They were slobs, I was the neat one - and once they were so mad at me about it, they let our gerbil loose on my bed - the gerbil was appropriately named "poopsey"....so I came to the room once with a bed full of gerbil poop!)
Sooner or later, they'll realize how to "get out of each other's way" and not bother each other as much. They'll go throught he "I hate you" shouting matches to the "She's not staying on her side of the room" and "she's ruining all my stuff." This is typical.
For what it's worth, and good luck!!