tmux: how to toggle "on" and "off" options with the same key

If you don't explicitly specify "on" or "off", the option will get toggled. The following would suffice:

bind-key a set-window-option synchronize-panes\; display-message "synchronize-panes is now #{?pane_synchronized,on,off}"

don't attach on or off. it's toggle bind-key a set-window-option synchronize-panes