(In fact, I've had a real problem getting tmux to behave well at all when multiple term emulators are connected to the same session. I've had weird window resizing problems.)
I wrote that in response to the last thread complaining about said feature came up.
tmux new-session -t <existing_session_name>
(Do a "tmux ls" to see existing sessions)
The one problem with this approach is that each session created this way (i.e. one that "attaches" to an existing session) actually creates a new "mirror" (?) session that will eventually clutter up your list of existing sessions.
My workaround is to create an initial named session (tmux new-session -s name), then attach to that named session. I never use "tmux ls" :p
You may also want to add "set-window-option -g aggressive-resize on" to your .tmux.conf (or prepend with "tmux" and run it in an existing session). To quote the manpage: "will resize the window to the size of the smallest session for which it is the current window, rather than the smallest session to which it is attached".
tmux a -d