My brother and cousin co-author a general humor blog, hosted on my server, and recently they complained to me that they couldn’t edit comments, which I’ve always been able to do in my own blogs.
Investigating a little, their problem was that they couldn’t edit comments in each others’ posts - even their own comments. Looking at the code, it’s written so that a user can only edit comments in other people’s posts if they’re of a lower level. In a way, it makes sense… but on multi-author blogs like that one, where both users have the same level, and trust each other, it’s better if you can also edit stuff from someone of your own level - including comments on their posts.
If you want to change it in your own blog(s), do the following: edit the file wp-content/functions-post.php. Search for the function user_can_edit_post. Inside that function, change the following line:
|| ($author_data->user_level > $post_author_data->user_level)
to
|| ($author_data->user_level >= $post_author_data->user_level)
Not rocket science, but it can save you a couple of minutes. 
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