See [this StackOverflow answer](http://stackoverflow.com/a/29757750/864684).

From [@killdash9](http://stackoverflow.com/users/544024/killdash9):

> Pressing d will copy both A and B to buffer C.

<!-- language: lang-el -->

    (defun ediff-copy-both-to-C ()
      (interactive)
      (ediff-copy-diff ediff-current-difference nil 'C nil
                       (concat
                        (ediff-get-region-contents ediff-current-difference 'A ediff-control-buffer)
                        (ediff-get-region-contents ediff-current-difference 'B ediff-control-buffer))))
    (defun add-d-to-ediff-mode-map () (define-key ediff-mode-map "d" 'ediff-copy-both-to-C))
    (add-hook 'ediff-keymap-setup-hook 'add-d-to-ediff-mode-map)