Can I use just half the biwire?

What lowrider57 said, +1. There’s no reason, what-so-ever, to let that cabling go to waste.  Just be certain to get the polarities correct.

I should have mentioned: I cannot double as they're bananas all around. I could get adapters for the dangle pair of course, but, does dangling actually hurt anything? 

If you want to use them that way put some shrink wrap or electrical tape on each dangling end to prevent an accidental short.

I’m just thinking off the top of my head and I use Purist shotgun biwire Museus cables but if you only connect it as a single run cable are you using only half of the wires capability? I’m thinking if you have a 12 gauge wire x 2 and you only connect one of them are you using only half of it’s potential? I'm thinking impedance and resistance differences here. Any feedback on this

@boss302 - That’s why Lowrider and I, responded as we did(ie: 2 x 12 AWG = 9 AWG).