We have one. Once or twice there was an incident where one girl was doing a knee drop handstand and the other one started bouncing, causing hyperextension of the handstand child's back. It was minor and transient both times, but I can see the danger. This is totally against our rules by the way. I do allow them to jump together, but if one wants to do a trick she must loudly announce it and the other must sit by the edge completely still. In the cases above, the child at fault was the one doing the trick - who started doing it unannounced while the other was already bouncing, so it was over in a blink before I could even say anything (I was right there). They were punished, hopefully they will remember. We have had it for 3 years and this is all that ever happened.
One other thing to note: I think the safety thing is also largely affected by how well-made the trampoline is/how much you pay for it, and how you install it, i.e., on level ground, anchored, etc. Ours also has a mesh overlap of 2 feet as a "door" rather than a zipper flap that can be left open. I have definitely seen some very rickety, unsafe-looking trampolines at friends' houses. Ours is designed very safely and is sturdy, never moved even through hurricaine Sandy, but all trampolines of course carry the risk of user stupidity, as my story above illustrates.
