I think that some question can start by one person offer bounty let say +50, and some other can offer +50 (total +100).
This way more than one person can share the cost of the bounty and at the same time show the interesting for a better answer.
This is an interesting idea, but it's functionally equivalent to having multiple bounties active on a given question at one time. And it's been explicitly stated in the blog that "Questions may only have one active question bounty at any given time."
A "merged bounty" would also have award issues; what if one contributor thought that user X's answer was the best, but the other contributor thought that user Y's answer was the best? What if they didn't contribute the same amount to the bounty?
I don't know if that would be a good idea. A 100 point bounty for 1 week is likely to see far fewer views than 2 50-point bounties for 2 weeks. I don't think the answers will necessarily be any better for a 100 point bounty, but having multiple bounties increases the question's exposure (over time), thus increasing the odds of getting at least one good answer.