How to communicate with another computer (on another network) terminal to terminal (OS X )
It's unclear what you really mean with communicate!

If you want to have a chat-like communication enable ntalk on both hosts:

sudo launchctl load -w com.apple.ntalk


Then enter

talk user@host


to open a chat session.

To enable the chat session on the other side just enter the displayed command e.g.

talk otheruser@otherhost


This might fail if the system versions of the two hosts are different.



To "mirror" a session/attach your Terminal window to another one on a different host use screen:

On host1/user1 enter

screen -S test #test is the sessionname and can be chosen more or less freely


On host2/user2 enter:

ssh -t user1@host1 screen -x test #use the sessionname from above


User1's input will be visible in user2's shell but is unable to execute anything on host2. User2 can execute commands on host1. Both users see all commands entered in both "shells".

Screen is a really mighty tool. Please consult man screen for further informations.