Only one person can be notified using @username per comment.
Adding an additional autocomplete would imply you can notify two people, when in reality you can't. Adding the autocomplete without adding notification of multiple people would be a bit counter-productive, but notifying multiple users has already been requested and declined.
In the meantime, what you can do is:
Make a comment:
Hey @Bob and John, thanks for the edits!
Edit the comment:
Hey Bob and @John, thanks for the edits!
Then both will be notified (and when you edit and delete the @ from in front of Bob, putting an @ will give you autocomplete for John.
