I want to use several emacs configurations/versions simultaneously so that no configuration disturbs each other, each with their own configuration directory and elpa repository.  I heard about the trick with symlinking `~/.emacs.d` and `~/.emacs`, but that is not what I want, because it'd be best if I can run my default emacs installation and *simultaneously* a testing installation (new emacs releases) or another setup like Spacemacs or Prelude.

Is there any decent way to do this?