create or add to ~/.tmux.conf:

# Toggle Pane Synchronization ON with Ctrl-x
bind-key C-x \
    setw synchronize-panes on \; \
    set-window-option status-bg red \; \
    display-message "Pane sync ON"

# Toggle Pane Synchronization OFF with Alt-x
bind-key M-x \
    setw synchronize-panes off \; \
    set-window-option status-bg green \; \
    display-message "Pane sync OFF"