The best way to do this without causing problems is to provide for multiple, separate bounties on a single question. Each bounty will run separately and will be controlled by their respective users. This ensures that concurrent bounties don't conflict with each other. Some limits need to be imposed, though:
- The total reputation offered by concurrent bounties should not be allowed to exceed a value like +750.
- There must not be more than three bounties at any given time on a question.
- It may be necessary to combine the post notices for each bounty together to prevent post notices from taking up an excessive amount of space between the question and the answer area. The text specific to each bounty may have to be hidden behind a "show details" link to avoid clutter.