As I only use kill-ring-save after I used set-mark-command, I wanted to try using its binding also for another command. However I'm not able to get this to work with an additional if. I don't know how to pass the arguments to avy-goto-char-2 with the interactive stuff.
How can I determine if the mark is active, and use that information to pick a function to call?
The function region-active-p will tell you if the region is active. You can use this to determine which function to call. Both of the functions you want behave differently when called interactively vs when called as lisp functions, so you need to tell Emacs to treat them as interactive function calls with the (call-interactively ...) form. So:
(defun my-double ()
(interactive)
(if (region-active-p)
(call-interactively 'kill-ring-save)
(call-interactively 'avy-goto-char-2)))
