I think Marie is right, according to the task life cycle, http://docs.jboss.org/jbpm/v5.1/userguide/ch.Human_Tasks.html#d0e3368, you won't be able to make this. When a human task is completed, you can't go to uncomplete it, you have to complete a new task instance. You could make some hack, changing manually the task in the db?, and something with the task work item, but in my opinion it would not make sense.
Would not filtering the history query for multiple instances?
Regards,
Demian