In this example, you are both connected to the same session. Each client can't focus on a separate window. It looks something like this:

         +-------------+
         | tmux server |
         +-------------+
            /
       +---------+
       | session |
       +---------+
        /       \ 
  +---------+ +---------+
  | client0 | | client1 |
  +---------+ +---------+

- `tmux server` is the 'backend' process listening on /tmp/pairprog

- `client0` and `client1` are the 'frontend' processes connecting to the socket /tmp/pairprog.

- `session` is the collection of windows that you are using.

The issue is that the currently active/focused window is an attribute of the session, meaning that all connected clients are always focused on the same window. What if you want to have each client focus on a separate window? This is behaviour that you get by default in screen, but you have to work a little more for in tmux.

The script is just a simple way to use session grouping, which would look like this:

         +-------------+
         | tmux server |
         +-------------+
            /        \
       +---------+ +-----------+
       | session |=| session-1 |
       +---------+ +-----------+
            |           |
       +---------+ +---------+
       | client0 | | client1 |
       +---------+ +---------+

Note the '=' between the sessions. I'm using this to denote that `session` and `session-1` are grouped, meaning that they share the same windows. Since each client is connected to a different session, they can switch windows independently.