Just use tail -f and a bash function.

Put this in your .bashrc file:

    function talk { echo "$USER: $@" >> talkfile; }

then run:

    tail -f talkfile &

The & puts tail into the background so it continues running, and "talkfile" needs to be a file that both of you have write access to.

You can both communicate simply by using the talk function like any other bash command:

    talk whatever you want and it'll be written to talkfile

This works on Linux, not sure about Mac.

It's nice because it records what you say, so there's no need for the other person to be logged in to get your message, and you get a printout of the last few lines of conversation when you "login" (run the tail -f command). There's nothing extra to install either.

(edit, apparently say is already installed on OS X, so I renamed the function "talk")