I don’t know how common joint practices are in the NFL, but I can tell you most sincerely they won’t continue much longer. Yesterday at a joint practice in California, the Cowboys and Raiders got into a massive brawl that spilled over into the crowd. At one point, a fan hit a Cowboys player with a helmet. He responded in kind with what appears to be a punch. This was like a tame version of the Palace Brawl.
According to Deadspin, the fight was one of many that broke out during practice. Here’s another angle from the crowd. Definitely looks a fan jacks #20 in the head.