Thomas -
Miror-Mirror cascade is supported in 8.3.2, but I don't think it's in System Manager.
You'd want to go to the command line.
You may want to review the Data Protection Guide:
I hope this response has been helpful to you.
At your service,
Eugene E. Kashpureff, Sr.
Independent NetApp Consultant http://www.linkedin.com/in/eugenekashpureff
Senior NetApp Instructor, FastLane US http://www.fastlaneus.com/
(P.S. I appreciate 'kudos' on any helpful posts.)