(these assumptions are based on StackOverflow.com)
Currently, a bounty may be placed on questions, if the question is (a) two days old and (b) some member has earned the bounty-privilege. After someone set a bounty on a question, it is not possible to increase a bounty for the questioning user.
Example: User A asks a question. User B has a similar problem and adds a +50 bounty. Now, User A would like to set an own bounty of +200, as the question has become very important for him, but can't, as a bounty has already been set.
So, the idea is very simple: Allow the adding of bounties by different users or bounty pooling. If there are several users, who'd like a solution for a question, they can increase the already given bounty by an own bounty.
Example (with bounty pooling): User A would like to make the question a +200 bounty question. As User B already added a +50 bounty, he stocks up the bounty by adding a +150 Bounty himself. User C also has the same problem and stocks up the bounty by another +100, making it a high-value bounty of 300 reputation in total.
The bounties should be listed with the user below the question, so that the bounty-givers may be identified. By adding a bounty, the bounty-giver may give an own bounty deadline. The system would perform the regular process at the bounties deadline (give to solution or the answer with the most up-votes).