I would like add more bounty to this question to have it more likely answered and signal my additional interest, so to say Superbounty / Multibounty.
But someone else set a bounty, which puts a lockout for additional bounties.
The easiest solution might be to let users just add bounty leaving all else equal (original bounty offerer may still decide, or bounty decision split).
By the comments below, I like to add following example: "A user awards 50 bounty to a question. 10 other users like the question and think it deserves more bounty so it be answered more likely." -> Currently, the bounty would be 50 while actual interest were >200. So this question is 'bounty-undervalued'. If every user can only award 50 and needs to wait for a week for the previous bounty to vest (as 'serial bounty'), the question may remain unanswered and undervalued for entire time. Having questions with higher bounties collectively would make them more attractive and they get more attention and better answers.
It was suggested that a Serialbounty could be better than Superbounty. I think there is no problem to let each user decide which bounty he wants to add; the fact that one cannot currently add his bounty if he thinks the question deserves it seems missing feature.
Likewise, any question can currently be upvoted by unlimited users, but it cannot have multiple bounties.
Can you please add the feature for Superbounties?