I yelled at mine the first time I saw them locked in what looked like a death grip. ALl that happened was that if they were playing, and I came into the room, they would both look really guilty and stop what they were doing. I found out that cat play is really rough looking. But they love it.
The rule of thumb that I use, based on advice here, was to listen - if one cat is squealing or crying to get away and can't, then you may intervene. And watch their ears, if one has its ears laid back on its head, and is growling or hissing, then its time to break it up. But don't yell, just make a loud noise or something to distract them.
I love watching them play. They grab each other, kicking, and with their mouths open trying to bite. But if one cat stops, the other goes right back to start up again. They love it.