cant really be done with http, doesnt work like that. if user1 made a request you could check to see if there are any messages (whatever) and envoke a popup then, but you cant just send stuff to a client.
ps: just had a bit of a thought. you could maybe get away with it using ajax. just setup a javascript timer to check for messages every few seconds, if there is a new one, envoke a popup. not sure how compitent you are with javascript, but the ajax theory is pretty simple. there is a brief tutorial here. not really what your looking for, but proves it is possible.