Consider multiple PM’s if you want any additions, deletions, or changes in one form not to be present in any other form until you save the first form.  In addition, you can do an Undo on one form independent of the state of another,  The disadvantage of using multiple PM’s is that managing them can be complex.  Moving changes from one PM can be tricky (for example, you’ll have to deal with possible concurrency conflicts).

Finally, there is the hybrid approach.  View the TechTip which uses theGetEntityGraph at

http://www.ideablade.com/techtip_saving_by_form.htm

Now that I have confused you, I would strongly recommend that you start with a single PM.  If after you have success with this approach, you might look at other alternatives.