receiver as amp
There are various ways to to do this that achieve the same basic result. Essentially you use the second receiver as you would an amplifier by sending it line level signals and using it to drive two of the speakers.
One way to do this is to send the pre-outs of the left and right main channels from the primary receiver to a cd input, for example, of the secondary receiver. You would then need to use a db meter, or an attentive ear, and set the appropriate volume level for the secondary receiver at listening levels to balance it with the rest of your system.