They don't talk about it, because--if you hadn't noticed--they never talk to each other about anything if they can help it, unless it is to insult or snark at each other.
So Beca just leaves the Res Ed housing form with her messy sprawling signature on it on Kimmy Jin's desk before leaving for Bella's practice. When she gets back, the form's sitting on her desk, with Kimmy Jin's small, neat signature directly below her own. The next day, she hands in the form to Res Ed, and that's it: she and Kimmy Jin are going to be roommates again their sophomore year.
It's not as if she could room with one of the Bellas, after all. The constant strain of having to be around someone she actually likes (or at least has to pretend to), whom she has to be friendly with, is more than Beca would be able to handle. Just imagining it leaves her feeling exhausted.
Kimmy Jin is safe. Beca doesn't have to constantly be trying to make the other girl like her, because the two of them don't like each other and are perfectly happy that way. She doesn't have to use up her reserves at being social, reserves which are pretty much used up just between the Bellas and Jesse alone; heck, once Beca and Kimmy Jin spent an entire two weeks without saying a word to each other, in complete, wonderful, comfortable silence.
And when Kimmy Jin finally did break the silence, it was with: "Stop being such a bitch, Mitchell." Music to Beca's ears.
Where is Beca supposed to find another roommate like that? More to the point, how is she supposed to find another roommate like that without having to actually talk with anyone?
It's a no-brainer, really.