Possibly dumb question: can I use two cable runs for each channel, not bi-wired?

You will be fine. As you noted you are producing what cable makers call shotgun cables. They double the wire and either combine them in two terminations or four terminations. I would just mark the end of each wire with a color tape to make sure you don't short anything out
Alan

I use double-cables two ways: 

a) For amps that I've owned having two sets of speaker outputs (Adcom and Krell) I have bi-wired by using both of these outputs and two sets of cables to bi-wirable speakers, like my B&W805D. 

b) For speakers that are NOT bi-wirable (my Thiel CS3.7) I run two sets of speaker cables and double them up on the speaker terminals. I'm doing this with two sets of Nordost Heimdall cables that I made from a single, really long run. I found that this added a little something to the sound. I think the additional cable mass makes a difference. Perhaps this is in line with Nordost's move toward more cable mass in their newer models. 

I am told by Ron Hedrich of Marigo Labs, legendary (in some circles) designer and engineer of cables and resonance/vibration control thingies, that it works, but make sure it's the same exact constructed cable. Also as stated by others, spade banana at each end. Which sounds like what you are wanting to do. Marigo also said it would make for better sonics to use biwire cable. With wire running cable length, not just attached at the speaker end. Hope this helps.

True bi-amping of speakers from different amps to different drivers, ie woofers and mains, would show a doubling of resistance when adding them together, which is not the case, because they are still the same to each set of drivers. Bi-wiring accomplishes nothing. That means that no improvements or degradations will be heard,k unless, of course, the wires were too small or too resistive to begin with. Simply doubling wires to the same terminals does not double the impedance. It halves it. However, since he mentioned he was using shorter wires, for example say to half the original length, the impedance would be the same as before, and for other lengths, easy calculable with simple multiplication and division skills.

Simply doubling wires to the same terminals does not double the impedance. It halves it. However, since he mentioned he was using shorter wires, for example say to half the original length, the impedance would be the same as before.

Dan, I’m not sure I follow the last quoted sentence. Cutting the length in half would cut the resistance, inductance, and capacitance in half. If the wires are then doubled to the same terminals (i.e., paralleled) that would cut the resistance and inductance in half again, to 1/4 of the value of the original single run that was twice as long, while doubling the capacitance to the original value.

Regards,
-- Al