I would never edit an answer to add new content. Editing should be reserved IMO for corrections like grammar, spelling, major character names misspelled etc. I recommend that you mention the points the other answer contains and attribute it to the user that answered.
You could consider this question which was answered by someone and accepted. But I felt compelled to post my own, since I felt something was missing. I acknowledged the accepted answer and the person who answered in my answer.
Or should I comment on the answer with the additional info?
You could add a comment if you think your comment could add to the answer, but by itself can not be a complete answer.
or should I answer again with the combined knowledge to sell myself out to rep?
I do believe if it is rep-whoring, we would notice something like that. And the community would react. Check out this question asked by our esteemed agent86. I thought it got closed for rep-whoring, I did comment as well. Not for the rep but for the hat. But did get re-opened again.
On SO it is not uncommon to see someone post
@User was right, in addition to what he said, but here is a caveat as part of a person's answer.
I personally do not think Arqade works quite the same way. We are different from most other SE sites in this regard.
Ultimately as @MBraedley put it, it depends on a case by case basis.