Can I "double-wire" speakers?

If you are trying to use the two amps as two seperate systems driving the same speakers,no. The top connections are connected to the mid/high x-over and the lower ones to the low x-over.

I agree with Unsound. Not only is it unlikely both amplifiers would have the same gain, but they will also have different characters and this will occur at the crossover point. Speaker manufacturers make every effort to develop a seamless crossover. Why mess it up? So, if you have an Onkyo receiver and a Sophia Electric the answer is no. If you have two identical amplifiers the answer is yes.

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

Use one amp and bi wire. I use purist maxinus bi wire for my mids and highs and bi wire purist elementa for my bass. Two sets of bi wire from Krell to B&w speakers. Basically, I have one amp and speakers quad wired. Using two amps that are not the same will create a muddy sound signature. Like speaking American English and British English at the same time. Be carefull, take you time to set up. And make sure everything is powered down.