I think that the main issue that you're having is that 'active window' is an attribute of a session, and not of the client. If 'active window' were specific to a client, then two clients could switch windows independently while connected to the same session.

You could petition for an option to the `attach-session` command that automatically creates a new session grouped to the session that you're 'attaching' to. IIRC, you can't attach two 'clients' to the same (GNU) screen session unless you use the -x option, so it's not like it's the 'default' behaviour for screen either.

Edit:

I wasn't thinking, but `new-session` will automatically connect to the new session you created, so there is no need for an option to `attach-session`. So when you want to connect a second client, just:

  # find the session you want
  $ tmux list-sessions

  # create a grouped session
  $ tmux new-session -t SESSION_ID

Edit 2:

  | This is so much more complicated!

Depends. If you're using byobu, then you're already attempting to abstract away a bunch of the complexity of GNU screen, so it's not like you would find less complexity if you dove into the screen internals. It's also this design that makes it easier to do complex things that screen can't do with its windows.

  | When using byobu I use F3 and F4 to cycle to
  | previous and next window.

Maybe you're misunderstanding. You still use the same tmux bindings to switch windows, it's just the setting up of the session that requires slightly more work when connecting a second client.