Can I "double-wire" speakers?

I doubt that both amplication sources will have the same gain.

...Or find out what the gain of the Onkyo and Sophia Electric is. Though there are exceptions, I think Rrog's advice is sound.

Bass is an integral part of music, that's why we think we need it.

Is the Sophia Electric a power amplifier or an integrated amplifier?

As it pertains to this discusion, I would consider it a one input integrated. In that case Popehetor's post makes some sense, though I still think Rrog's post makes sense. Cross-overs will still allow these different amps to overlap somewhat. Juggling the volume controls on the different amps all the time to match gain, is not something I would look forward to.