What does this warning mean?

Question

When I try to mention 2 users in a comment, I see something like this:

Only one user can be tagged. The post owner will always be notified.

Answer 1

It means, If you comment on a question or an answer, the person who posted said answer or question will always be notified, even if you don't tag them. So there is no need to tag them.

You cannot tag more than one person in a comment. It's designed this way, since it's hardly ever necessary and mostly used to spam mention everyone in a comment thread. If you really must, leave two comments, but first ask yourself if you really need to, and whether inviting the users to chat isn't more appropriate.

More info on comments can be found on Big Meta:

Can I notify more than one person at a time?

No. Comments containing more than one @name are blocked unless they contain a backtick `` . In the latter case, only the first name mentioned using the @name syntax will be notified. For example, @alice @bob Hi! will notify Alice (if she has participated in that post), but not Bob.

An exception is the case when the first @name either matched nobody, or matched the post's author (and thus isn't necessary); in this case, the next @name will be checked.