Congratulations on your acceptance!!!!!
My roommates and I did this very thing - met online then requested we be in the same apartment - there were 4 of us.
This is what we did...this was over 3 years ago, so I'm sorry if it's changed somewhat...
1) On our online DORMS application, I'm pretty there was a request section or someplace to link applications of other people you wanted to room with on it; we did that. We then made sure we all had the same things selected that we could - like apartment location, wellness vs non-wellness, number of roommates, ect.
2) On our actual arrival date, we met up in the line before the whole process started and basically did everything as one solid group. Once we got our job details, we went together over to the Dorm selection phase and talked with the guy there. We told them our names, that we all wanted to be in a 2 bedroom, preferably at Chatham, and boom it was done. They gave us our keys, took our photos for our IDs and that was it.
Now as for the odds of it happening, not sure. My group was the only people I met that had picked out roommates ahead of time. Everyone I met on my CP did random roommates. I can tell you, though, out of all those people, ours was the only apartment that no one had any complaints. I can't tell you how many times I'd get to work and hear others complaining about a roommate or something that happened at their apartment.