Using Onkyo TX SR703 to bi amp Speakers

I am currently using a Onkyo TX SR703 AV receiver to drive my Vintage Polk LS90 towers. This is a temporary setup until I can get my vintage Denon Equipment up and running. 

I am am just using the front 2 channels for stereo and my question is I my thinking of bi amping the Polks using the rear surround channels and putting the listening mode into “ALL STEREO “. In this mode all 4 channels will receive the same input. 

The newer Onkyo AV receivers have this as a setting in their firmware to do this and make the receiver a 5.1 setup. Since I am runnng only the 2 speakers I believe this should work. 

The Onkyo is rated at a probably inflated rating of 100W per channel x 7. 

The Polks are 8 ohm. 

Anyone have experience with this.