Can I use just half the biwire?

Don't leave them hanging. Double them up and use them on the speaker binding posts. You'll benefit from all the conductors being connected.

I don't know if it's kosher, I would say not, but I had a set of biwire cables that I used on my main speakers, and when I would occasionally switch to a small pair of Celestions that were single wire, I would use those cables with the unused set dangling, and I had no ill effects. 

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.