Can a comment be addressed to multiple users with the "at" sign (@)?
No, it can't. Only the first user mentioned with
@, and also the author of the commented post, will be notified. Note that the user must have been commented on the same post before.
An exception is the chat which works different. Here you can have multiple
@user in one line. This works as long the mentioned users visited the particular chat room not too long ago.