It really depends on what your workflow is. If you only ever have a single tmux session, then you could write a quick script that starts a session if it doesn't exist, but if it does, automatically creates a mirrored session and connects to it. In this workflow, you can assume which session you want to group with, but if you work with multiple sessions, then there is no way to automatically do this. E.g., you could run something like:

  $ tmux.sh session-name

which would create the session the first time, and create a grouped session, the second time.

Edit:

I've created a little script for this: https://github.com/bsandrow/tmux.sh

I created a byobu ticket to cover this issue as it is what ultimately starts/connects tmux pointing to this thread. Thanks for the solution. https://bugs.launchpad.net/byobu/+bug/1168734