The Back Amp Channels can be configured either for 7.2 speakers, bi-amping or Zone 2 use.
When configured for Zone 2, the back amp channels cannot be used for bi-amping or 7.2 in the main zone. A 5.2 speaker setup is the maximum speaker configuration in the main zone when using the zone 2 speaker output.
If you want to run 7.2 speakers in the main zone then you must use the Zone 2 Pre-out or HDMI connection for Zone 2.