Getting your application to work properly using multiple persistence managers is much harder than getting it to work with a single persistence manager.  It is certainly possible to use multiple persistence managers, but it is usually a lot of work, and you must be prepared to do a lot of debugging yourself.  Also be aware that we have almost no documentation on this subject.  

I would also comment out that I know of no customers who have adopted using multiple persistence managers as a general practice, although I do know of several customers who have tried it. . There was one large customer who got multiple persistence managers to work, but they entually abandoned the practice as it was too complicated to maintain.