See this StackOverflow answer.

From @killdash9:

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

(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)

See this StackOverflow answer.

From @killdash9:

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

(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)