[Only one person can be notified using `@username` per comment](http://blog.stackoverflow.com/2010/01/new-improved-comments-with-reply/).

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](http://meta.stackoverflow.com/questions/45273/allow-more-than-one-name-notification-per-comment).

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.