You can't connect ssh clients like you would modems. If the machines are on a public network you ssh to another person's sshd, then use terminal chat programs. If you're both behind a NAT, one of you needs to port-forward to your host's sshd, or use STUN, TURN or ICE servers, or maybe just IPv6.

Once connected to someone's host, use the Talk program (https://en.wikipedia.org/wiki/Talk_%28software%29), or the Write program (https://en.wikipedia.org/wiki/Write_%28Unix%29), or use Netcat (http://hak5.org/episodes/haktip-82) to open a two-way dialogue between terminals. Netcat is the simplest of them all because it just opens a two-way tcp session, and technically only one of you needs netcat while the other just needs a telnet client or equivalent.