You can bind this to your own key using something like:
(global-set-key (kbd "C-x C-o") 'ff-find-other-file)
But of course you’ll need to pick a key that doesn’t already have something useful bound to it 🙂
Check the excellent Emacs-fu article Quickly switching between header and implementation
In case the blog post has gone, here is copy of the solution:
(add-hook 'c-mode-common-hook (lambda() (local-set-key (kbd "C-c o") 'ff-find-other-file)))
Now, we can quickly switch between myfile.cc and myfile.h with C-c o. Note the use of the c-mode-common-hook, so it will work for both C and C++.