"I can't run speaker cables from the amp-- they are
taken up by the bi-wired speaker cable."
You just parallel another set in with what you have now.
That's how you usually do it. It won't hurt anything as
long as you don't have any stray/frayed wires that could
touch each other.
If you are using banana plugs with your bi-wire set, use
spades with the sub wires. If you are using spades with
your bi-wire set, use bananas with the sub wires.