If you connect the amp to the surround back channel you will only send the back surround signal to the speakers in the other room which is not what you want. If the speaker backs can be set for zone 2 to you would have to go into the setup menu and change the configuration every time you want to use the speakers in the other room.
Since you do not have any line level outputs on your receiver you will need to connect a speaker to line level adapter like this
http://www.walmart.com/ip/Scosche-LOC80-LINEOUT-CONVRTR-ADJ-80W/10756155
to the front speaker outputs. Then you can connect both the left and right speakers from your 7.1 channel set up to speaker A and the other room speakers to speaker B. You will need to select stereo or multi speaker stereo when you use the other room speakers otherwise you will not get any voices which are being sent to the center speaker. You should benefit from using the amp on your front speakers in the 7.1 channel setup.
A true 2 zone receiver would have made this easier and would allow for different sources to play in each room.