The system only allows a comment to have only one @name in it. Unfortunately if this is a meta discussion about how notifications work, of the discussion about not deleting @postowner from comments, then you get an error message saying only one user can be notified.
Since it is common to use
@postowner in these comments, please make a special case exemption for that specific literal use.
Addendum: to be clear I am talking about the literal string "@postowner" not a @ directed AT the postowner