I would expend a lot of energy and time (and a little bit of money) trying to repair/replace that wire before trying anything like that. I don't think your solution would work the way you described. [EDIT - I stand corrected - see ArnyK below] Running them both from the a single wire pair in mono is probably the best available option without running a new wire, and even that is risky for both your speakers and amp.
How long of a run are we talking? Is the wire tied down within the ceiling/wall, or is it loose? If it's loose, just securely attach a new wire to the old, and pull the old out from the other end until the end of the new wire pops out the other end. If you can access the wire at any point in the run, you can use a multimeter to figure out whether the break is on one side or the other and splice in a new section of wire to bypass the break. [EDIT - I'd still try to run new wire or repair if you can, but good to know there's a backup option]