edit: sorry but my workflow is this: service-humantask(john)-humantask(mary)-service
If I understand correctly, you want to have John and Mary complete the same tasks for the 3 instances you have started.
In that case, the calls to taskService.getTasksAssignedAsPotentialOwner should return multiple tasks. Just loop through those to start and complete them.
then if I make: TaskSummary task = list.get(0); then I take the task for the first processInstance started???
and then with list.get(1) I take the task for the next process?