Similar to "How about making it possible to set up bounty on someone else’s question?", but a little less extreme..

Basically, you could "top up" the points of another users bounty (up to the current maximum, 500)

Say a question has a bounty of 100 points, I could contribute up to 400 of my points towards it it. When the bounty is resolved, 100 points would be deducted from the original user (well, 50), and 400 from me.

More than one user could contribute (again, up to the maximum limit)

If the question already has a 500point bounty, this feature wouldn't apply.