I would like to use @dev @prod in comments sometimes. @dev is not a user, it is a drush server alias.
I got the following error message:
Only one additional @user can be notified; the post owner will always be notified.
If you write "
@dev" you don't get any error message. Differently, Stack Exchange will think you are referring to a user, and it doesn't allow you to refer to two different users in that way.
The problem is that the software doesn't check if what written after @ is a username, or not; this means that you get that error even if there aren't users who use "dev" as username.
The alternative is using the backslash, but that would be rendered as
\@dev, in comments.