I suggest allowing an answerer (user A) to give part of credit to another user (user B) for their answer. A would specify the amount of credit as percentage (C). B would appear as a co-author of the answer. The reputation bonuses for the upvotes of the answer would be divided between A and B:
- A gets 100% - C%
- B gets C%
This would be used when A used an idea of B to build upon, B made a valuable improvement (edit) of B's answer, and so on. In these cases A would be able to give a proper credit to B.
This feature may also encourage users to improve existing answers by edits or comments and may even reduce the number of redundant answers (people would improve existing answers instead of posting the improvement as a second answer, rendering the existing answer redundant).