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 500 point bounty, this feature wouldn't apply. Also as cletus suggested, you should not be able to add to the bounty within maybe two days of the bounty closing.
Updated Suggestion: Acknowledging the inherent complexity, what if, when offering a second bounty, the user immediately agrees to forfeit control of who the bounty is given to <-- the original user to offer a bounty does so, and further, the bounty expiration remains unchanged. This would help lower rep users. They might not be able to offer the astronomical +500 rep bonuses that the 10k crowd can. These +500's certainly attract more views than the +50s. Presumably, the 10K users also already have the ear of the Stack Exchange Gods and need not beg so arduously for a feature on Meta, or have the visibility on [so] that their questions are answered quite quickly. Right now, if they forfeited too much rep it would relegate them to maybe barely being able to talk in chat. They might lose: 'create tags', 'retag questions', 'view close votes', 'reduce ads', 'vote down', 'create chat rooms', 'edit community wiki', and 'set bounties'. While none of these are huge, it also makes that hope of someday getting to 10k appear infinitely further off in the future...
As an example, this proposal is reasonable, maybe not stellar, but reasonable, so a low-rep user could offer a larger bounty in the hopes of getting on the "someday-we'll-implement-this-when-Jeff-gives-us-Friday-off" list.