Pulled up the PDF manual on this unit, and it has a line that states:

IF another pre-main (integrated) amplifier is connected, the ZONE2 jacks can be used to play a different program source in ZONE2 at the same time.

Meaning? Yes, you really should get another amp/pre-amp for the second zone. Using the on board surround sound back amp just isn't a good way to get where you want.

And, for what it's worth, you don't really need a IM volume control in this case. It certainly won't hurt, but it's not needed.