When there is a discussion in comments (especially on meta), sometimes I miss an opportunity to call two users in one comment. Hence, when I need to answer both users and I want to be sure that they will receive answers in their inbox, I have to write two comments - which would be meaningless if only both users can be called in one comment. On the other hand, maybe I don't consider some negative situations which may be caused by such feature?
A possible workaround would be to put a separate comment just for the sake of adding a ping to a specific user; e.g.
@XYZ: Perhaps you could check the above comment, too.
Of course, if you prefer, you can delete the comment once you know the person you wanted to ping has been already notified.
Aside note: In the comments above it was suggested, that a ping from a comment gets to inbox even if the comment itself is deleted. Based on a little testing, it seems that this does not work. In a related thread at meta.SO: Is it OK to use temporary comments to notify users interested in a post? it is mentioned that:
I was wrong in my statement in point #1; Inbox notifications of comments that have been deleted do, in fact, get deleted from the Inbox, also... just not immediately. Additionally, deleting the comment immediately after posting it causes there to be no inbox notification in the first place.
In an answer in that thread another possible workaround was mentioned - to ping the user in chat. (However, this can be used only if the user you want to ping uses chat.)