Emacs command to open a file and its side car side-by-side

find-sidecar.el

(defun find-sidecar (file extension)
  "Open FILE and its EXTENSION sidecar in adjacent windows.
Prefer an existing window to the left, then one to the right."

  ;; Usage:
  ;;   M-x find-sidecar RET
  ;;   File: path/to/file.ext RET
  ;;   Sidecar extension: ext2 RET
  ;;
  ;; Opens:
  ;;   path/to/file.ext
  ;;   path/to/file.ext2
  ;;
  ;; The extension should not include the leading dot.

  (interactive "fFile: \nsSidecar extension: ")
  (let* ((active (selected-window))
         (left (window-in-direction 'left active))
         (file-window (or left active))
         (sidecar-window
          (if left
              active
            (or (window-in-direction 'right active)
                (split-window-right)))))
    (set-window-buffer file-window
                       (find-file-noselect file))
    (set-window-buffer sidecar-window
                       (find-file-noselect
                        (file-name-with-extension file extension)))))