You can use xcape under x11 to configure Shift to emit e.g. Alt+Ctrl+Shift+F12 when tapped and released. Then you would declare that as a secondary
tmux escape key, and assign a Key-binding to "Alt+Ctrl+Shift+F12" for
command-prompt "find-window -N '%%'"
The exact sequence of commands:
At shell prompt:
$ xcape -e 'Shift_L=Alt_L|Control_R|Shift_L|F12'
At tmux prompt:
:set-option prefix2 "M-C-S-F12"
:bind M-C-S-F12 command-prompt "find-window -N '%%'"
Now double-tap Shift_L, and on the bottom left of your
tmuxyou will see
Shift_R=Alt_L|Control_R|Shift_L|F11 you can use Shift_L for inner
tmux, and Shift_R for outer
tmux, if you nest them.
Note: I had previously posted this answer with F15, but that did not work as expected , because @Gilles says:
F20 need to be declared in