The notify list will only respond to the nicks that are in the notify list and on your currently active network.
So, using your example, Nick1 would only see nick2 if nick2 was on the same network as nick1, although the same person or someone else might be using nick2 on network2 at the same time
To check all of the networks that you're connected to for matches with the notify list, you might try
alias notify2 {
scon -at1 /notify
}
Using
/notify2 should issue the /notfiy command to all networks that you have an active connection to. NOTE: You may get false responses due to the reasoning I gave earlier.