As stated above, the two sub pre-outs on an AVR are normally for the main zone only therefore the Zone 2 sub will need 2 sets of speaker posts on the sub (ie. 1 set as "input" from the Zone 2 speaker posts on the AVR and 1 set as "output" to the Zone 2 speakers).
The AVR can be a 7CH model (ie. 5.1 + Zone 2).