This is a perfect example of the dangers of third parties
At least in a Parliamentary system, they hash it out at the beginning and arrive at a coalition that hangs together for however long it can.
Under our system, a strong 3rd. party insurgency would force something like this to happen on every piece of legislation that came along.
(of course, it that had the effect of stopping Congress from doing anything, probably not a bad thing)
Incidentally if there is some sort of Lib-Lab coalition how long would it be before another election or would this be some semblance of a stable parliament?