Comment 6 for bug 1032042

If those users each have their own account things should be fine. The interface is on the session bus, users in separate accounts would get their own local bus name org.onboard.Onboard and there should be no ambiguity or interference.

Running multiple instances of Onboard by the same user is not supported very well. Onboard is pretty much single instance at this point. Running multiple instances is still possible with the -m option, but in practice you'd run into trouble, since there is only a single set of settings, most notably for position and size.
The bus name org.onboard.Onboard points to the primary instance only, but I'm thinking of making each instance's bus address available somehow. D-Bus calls could be sent to individual instances then. Perhaps there is a way to query names by pid already, but I haven't found out how to do that yet.