I tried the first answer without success.
I was able to get the results I wanted by doing the following:
tmux attach
I entered tmux and saw the other session was still attached
So I detached my current session to get back to a shell: Ctrl + b; d
Then I issued the following:
tmux attach -d
This says to attach to the default session, and detach all other sessions currently attached. See the man page under Clients and Sessions
attach-session [-dr] [-t target-session]
(alias: attach)
If run from outside tmux, create a new client in the current terminal and attach it to
target-session. If used from inside, switch the current client. If -d is specified, any other
clients attached to the session are detached. -r signifies the client is read-only (only keys bound
to the detach-client command have any effect)