You won't be providing additional power, you'll be providing power in place of the onkyo's amps. In order to use a second receiver strictly as an amplifier*, you would need to run rca's from the preouts of the first receiver to the multichannel analog inputs of the second receiver (one rca for each channel). And if the second receiver doesn't have a multichannel analog input, then there's no way to use it strictly as an amplifier*. The red/white rca inputs you see are the regular 2-channel analog audio inputs. You could theoretically connect the preouts for your front left and front right channels to one of those inputs, but then you'd be using the second receiver as a receiver (pre/pro and amplifier) in addition to still using the first receiver as a pre/pro. So the signal would have whatever processing and volume control is being applied by the first receiver plus whatever processing and volume control is being applied by the second receiver. I can't think of a single reason you'd ever want to do that. And even if your second receiver does have multichannel analog inputs, the only reason to ever use two receivers in this fashion would be if one receiver has a far more capable amp section than the other (and i do mean far, as most receivers amp sections aren't different enough to make much real world difference).
*Technically, you're still not using it strictly as an amplifier, as the master volume will still be in the signal path. But at least you're avoiding any and all processing, etc. that way.