Install openSSH, and libnotify-bin (via terminal)
sudo apt-get install openssh libnotify-bin
on both computers.
Then SSH (via the terminal) into the other computer and when your logged in, type in:
ssh <user name>@<ip address>
and then when your logged in, type:
export DISPLAY=:0.0 notify-send "Title of message" "message text"
Hope this works, Jonathan.