Normally, the main left/right channels are almost a requirement on processors. This is because they are the "priority" channel. If you have center channel or surround channels disabled, the processor will usually mix in those signals to the left/right main outputs.
If you do not have any front left/right speakers, I would still enable and use the left/right outputs of the processor and connect them to the "side surround" speakers. Then take the normal surround outputs and connect them to the "rear surround" speakers.
It's not the best solution, but it will get you to where you want to go.
If you do not have any front left/right speakers, I would still enable and use the left/right outputs of the processor and connect them to the "side surround" speakers. Then take the normal surround outputs and connect them to the "rear surround" speakers.
It's not the best solution, but it will get you to where you want to go.