There is a command to switch to the last visited pane: ctrl-b + + ;
You can bind the # key (or anything you like) to switch to the last visited
tmux pane. I chose # because it's like how:b# switches to the last buffer
in vim:
# go to last pane (like b# in vim)
bind '#' select-pane -l
So you can just press you tmux prefix (by default ctrl-b) followed by #.