In ecology, I understand that first authorship is very important. That name becomes associated with the paper. Also, hiring committees, scholarships and impacts rankings weight first authorship as much more important than co-authorship.
However, if the paper only has two co-authors then the value of first authorship vs. co-authors seems at least somewhat less relevant For exampel, both names may become associated with the paper. I'm sure this question has come up, and would be happy to hear perspectives about it. I don't think that answers should be limited to ecologists, but it seems relevant if answers come from fields that similarly weight first-authorship as important.