Yes, you could technically do what you are suggesting and it would function relatively fine. You'd need to configure the older, more powerful receiver to act like a multichannel amplifier, which appears possible since it has a multichannel analog input section. So, turn off all processing (maybe put it direct or pure mode) and set all the speaker channel levels to "0" or the center-ish position and adjust the volume control to as high a level as you can to avoid any consistent hiss in the speakers.
However, what are you gaining by doing this?
Well, according to the specifications you are doubling the available power for your speakers. That means you can get an additional 3dB SPL from your speakers in the room over the smaller digital amped receiver. Do you need 3dB more acoustic output from each speaker? Maybe, but there are also drawbacks.
New issues could include more noise, more distortion, volume control imbalance, more heat generation in the room, more hum, and so on.
If I were you, I'd start by using the smaller receiver and tuning it up and seeing if it can provide enough power. If you want to test it, you could try using the larger receiver just for the main stereo speakers, or the front three speakers (the speakers which receive the vast majority of the power with nearly all soundtracks).
Ultimately, I think there would be very little improvement in the real perceived performance, if any at all. I think the issues of setting it up, the added noises and such, and the issue of dealing with another component generating heat and taking up space and constantly remembering not to touch the older/bigger receiver or else all the settings might go awry, make it not worth the effort.
If you need more power, just get the digital receiver and later buy a good stereo amp for the main speakers.
But, yes, your proposal would function fine once setup properly.
