Sometimes a few people all answer a question with subtly different but all useful answers. Wouldn't it improve Stack Overflow if we could merge the best answers and allow people to contribute to an already good answer and gain credit for doing so?
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This is a difficult one to quantify exactly, but I would say that the answer is almost certainly No.
Stack Overflow is about Questions & Answers, not about people. By that naive measure it is obvious that your suggestion is absolutely beneficial to the site. However, that would be ignoring the immortal words of Jeff Atwood.
Or, altenately, to paraphrase Charlton Heston
This means that the Questions & Answers are generated by the users, and we must understand their motivation. Each of them are volunteering their own time for their own reasons, and in many, many of those cases the reason is reputation. Whether for personal bragging rights or big numbers on their CV, they are trying to accumulate reputation. So, if merging the answers loses them their reputation, you'll see a fall off in participation.
Also, we already have Community Wiki for that purpose, and it isn't used often enough for exactly this reason.
This at least respects the problems illustrated in the A) section. However, it opens up two separate and very large problems.
As a result, apportioning the reward would be wildly unfair in nearly every case, and this feature would just cause frustration to both of the parties involved. Except for the people who game this system, they would love it.
On Stack Overflow, it is a little different, I would love it if people correct my answers, but in most of the questions I choose to answer, it is because there is still some way for me to make the answer mine. I want to be able to tell a prospective employer "Go to Stack Overflow and look at any of my answers, I stand by every one".
I accept (and embrace) that my answers can be edited, but if a user wants to change (or collaborate) my answer, then I would recommend that they create their own answer that incorporates mine, rather than they change mine. That way, if either of us so desires, we can go in a radically different direction with our edits, if we change our minds.
Bottom line: No.
There's so much perspective and context to different answers, that even though they may both be right, it wouldn't be easy to always integrate them. Just upvote them both, and they'll both float to the top and be seen.