I'm not sure if this idea has been discussed before, but I couldn't find any posts relating to it:
suppose Joey and Chloe are two users who identify post X as describing a problem of interest to themselves, who each wish to offer a bounty but do not posses (or are not willing to part with) the minimum amount of reputation to offer a bounty. Would it be possible to allow each user to contribute 25 rep to a pool which becomes a bounty on the question?
This could also be used for cases where the minimum rep amount can be met, but higher amounts are desired.
Of course, the standard prohibitions about collecting one's own bounty would extend to the group, which should not have anonymity.
As for the implementation, I can imagine a system whereby one nominates a question with an initial deposit of the reputation that the user wishes to contribute, after which further contributions are accepted for a period of time, and finally a deadline is reached when no further contributions are accepted and the bounty is posted.
Is there any reason not to implement such functionality?