ido-other-window

This plugin provides an alternative to the ido-*-other-window set of
commands. The following shortcuts will become available to everything that
relies on ido-completing-read:

  C-o - invoke in other window
  C-2 - split vertically and invoke in other window
  C-3 - split horizontally and invoke in other window
  C-4 - invoke in other window
  C-5 - invoke in new frame

It is the author and maintainer's strong conviction that this
functionality would make a nice addition to ido itself.