In this case, you could add a comment to the most up-voted answer that says not to forget what reported in Laura Scott's answer. I am not sure that is totally necessary, but it would be useful for who reads the first answer, and doesn't read the second one; in such case, it is also possible the same user doesn't read the comments for the first answer, but it is also possible that user reads the comment.
About the operations you suggest, I can say:
- Merging answers is not even an operation that moderators can do in a single step; that is because it is an operation that in few cases is justified.
Even in the case the two answers are given from the same user, merging the answers is something that should be done, as giving two different answers is perfectly legitimate. If a user is giving the same answer, but using different words, then one of the answers can be deleted. Normal users should down-vote one of the answers, in that case, as the user is doing anything useful, by writing two different answers that are both giving the same suggestion to the OP.
Merging two questions given from two different users falls in the same case of editing an answer to add something not reported by the user who answered. In the case of merging (if that would be a possible operation), you would even take off reputation from a user who wrote an answer. (If the user didn't write an answer, such as the case of a user writing a question about the question or one of the answers, then the answer should be flagged as "not an answer" which is a different case.)
Editing the answer given from a user to add something that user didn't say is not something that should be done, as there are guidelines about editing the post written by other users, and those are the following ones:
- fix grammatical or spelling errors
- clarify meaning without changing it
- correct minor mistakes
- add related resources or links
- always respect the original author
If you are adding something the user didn't say, you are changing the meaning of the post, which is contrary to what reported by the second point.
If you were adding to the post something said in a comment from who wrote the post, that would be different, and it is normally done with questions when the OP reports for which Drupal version is interested in the sixth comment for that question. In that case, what added to the post needs to be very relevant to the post; if the comment was something like "I really hate theming in Drupal." that would be not much relevant for the post, and should be left as comment (and possibly flagged as off-topic/not constructive). It should be also something that is stand-alone, which means can be understood even when taken out from the context of the previous comments; a comment such as "@user-who-previously-commented You are wrong." is not something that should be written in the post.
- Accepting an answer is something that only the user who asked the question. The community way to say "this is the accepted answer" is up-voting the existing answers; the most up-voted answer is the one that the community accepted. In this case, there is already an answer with 15 up-votes.
You could write an answer, and make it a Community Wiki as Clive suggests. I agree with the suggestion to make it a Community Wiki, as you are writing your answer basing on what reported by other users. Before writing your answer, keep in mind that:
- Your answer should report something more than what already said in the other answers; differently, it would not be that much useful.
- Your answer would be listed as last answer because the ordering normally used by Stack Exchange sites is by votes. It would be prominent if it gets more than 15 up-votes, or the OP accepts it.