Do two different comments. If you try to do two @mentions in one comment, only one user will be notified.
Can I notify more than one person at a time?
No. Comments containing more than one @name are
unless they contain a backtick
`. In the latter case, only the
first name mentioned using the @name syntax will be
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.
Can I change who gets notified after the comment is posted?
When editing a comment within its limited editing period, if you
change or add @name, the notification may or may not reach the new
depending on timing.