I'm not quite sure what you mean by 'projects' but I assume that it is two different
JBoss instances or something that are running in two different JVM?
I suppose that their could be some ways to get and start an user-transaction remotely by the client so that you could roll back or commit as needed (a little bit depending on the underlaying database also).
But maybe the simple answer would be to avoid that and keep the two EJBs in the same context so that they can share the same (EJB) transaction.