i have the same problem, i tried to assign more actors to a task, but i cannot get back a task or a taskinstance for one of the pooled actors.
ok, i tried to get back the taskinstances which were assigned for many users, but it didn't work at first.
so i tried to get the taskInstances for which my actor is pooled for, there i got the needed instances.
List taskInstances = jbpmContext.getContext().getTaskMgmtSession().findPooledTaskInstances(actorID);
after that it was easy to set the actorID for the instances, that's what i wanted to do.
why dont u use the fork tool... it will spillite ur transaction into two distinct tokens,,, then u can assign to each one different user with the ordinary way using swinlanes or what ever