Can I "double-wire" speakers?

You sure can. Bi-amping is a common practice that will really let those speakers sing. Let use know what you think.

STOP!!! Let's clarify this first. You can only do this if you REMOVE the jumpers on the speakers and have one unit driving the low frequency posts and the other driving the high frequency posts. Do NOT hook up each unit to the same posts. This would be very bad.

I would remove the jumpers on the speakers and have the Onkyo hooked up to the low (bass) posts and the Sophia hooked up to the high (tweeter) posts.

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