My perception is that the Philadelphia programs do a better job than most in working together.
The NYC programs occasionally have lectures as part of a NY PM+R society. I think they meet once a month.
The 2 UMDNJ programs have some cross-over. They meet 1-2 times yearly for the NJ PM+R society. The JFK residents do their spinal cord rotations at Kessler, so they integrate through that. They also share some courses together, and most of the JFK residents come to the Kessler review course. By the end of the 3 years, most of the UMDNJ residents know each other pretty well.
There are also some pairings that occur because of friendships amongst attendings. For example, Mike Geraci from Buffalo has a close relationship with both RIC and Michigan St, and lectures at both facilities frequently. This creates a bit of a relationship between the programs.
Similarly, I was recently hired at Arkansas to run their sports and spine program, and my close friend was hired for the same position at the University of Kansas. I anticipate that this will create some level of partnership, or at least familiarity, between the University of Arkansas and the University of Kansas programs