I'd say, if you can enforce the one at a time rule, so much the better. Maybe that's easier with slightly bigger kids. Mine are 2 and 4 and no sooner have I zipped them in than they want to be out if they're alone.
I do let them go on together because ours is a biggy, with nets, but I make DD bounce very gently, never with her back to DS. Mostly they just roll, chase and cuddle. I used to be okay with this but I can't say I'm comfortable with it now because, recently, 2yo DS got his tummy bounced on, full bore, by a 6' dad who got on the bouncy castle when DS was on, and I couldn't get him off in time. His bounce 'pulled' DS into the hole he made IYSWIM and then he landed on him - full foot, right across his tummy. It probably doesn't sound so bad unless you saw it, but seeing it made me feel sick and I took DS to A&E to have him checked out.
If you can't stick to one at a time, I'd stick to 2 evenly sized ones, sticking to bouncing, with no fancy moves, and make sure they've got strict rules and you're keeping an eye on things.
I'd agree with ABetaDad re size. If DH or I go on with kids we go on all fours as ChasingSquirrells says. It's more a case of a roll about and a cuddle etc.
Eastereggfeaster, we got ours last year and its one of the new Jumpking Ovalpods. As its name suggests, it's oval and it's supposed to have 2 bounce zones, one at each end, (or maybe 3 if this article is to be believed ezinearticles.com/?Jumpking-Ovalpod----Why-Oval-Is-Better-Than-Round-Trampolines&id=1184132) so it's supposed to be safer for 2 kids - if you can get them to bounce at each end!