default.nix

let
  rawfunc = import <nixpkgs>;
in
with rawfunc {};
rec {
  imagemagick1 = pkgs.callPackage ./imagemagick-old.nix {  };
  imagemagick2 = pkgs.callPackage ./imagemagick-new.nix {  };
  env1 = pkgs.runCommand "env1" { buildInputs = [ imagemagick1 ]; } "exit 1";
  env2 = pkgs.runCommand "env2" { buildInputs = [ imagemagick2 ]; } "exit 1";
  yourscript = pkgs.substituteAll { src = ./yourscript.sh; inherit imagemagick1 imagemagick2; };
}

imagemagick-new.nix

{ stdenv, fetchurl, python, pkgconfig, zlib, glib, flex, bison, makeStaticLibraries, glibc, libtool, bzip2, libX11, libXext, libXt, fontconfig, freetype, ghostscript, libjpeg, lcms2, openexr, libpng, lib\
rsvg, libtiff, libxml2, openjpeg, libwebp }:

let
  cfg = {
    version = "6.9.4-10";
    sha256 = "a2c981ea80de0ed95135591d0e504a2427187594a5cb2342474f50d97e626a2d";
  };
in
stdenv.mkDerivation rec {
  name = "imagemagicknew";
  buildInputs = [ python pkgconfig zlib.static myglib flex bison glibc.static zlib fontconfig freetype ghostscript libpng libtiff libxml2 ];
  inherit (cfg) version;
  src = fetchurl {
    urls = [
      "mirror://imagemagick/releases/ImageMagick-${version}.tar.xz"
      # the original source above removes tarballs quickly                                                                                                                                                  
      "http://distfiles.macports.org/ImageMagick/ImageMagick-${version}.tar.xz"
      "https://bintray.com/homebrew/mirror/download_file?file_path=imagemagick-${version}.tar.xz"
    ];
    sha256 = "a2c981ea80de0ed95135591d0e504a2427187594a5cb2342474f50d97e626a2d";
  };
}

imagemagick-old.nix

{ stdenv, fetchurl, python, pkgconfig, zlib, glib, flex, bison, makeStaticLibraries, glibc, libtool, bzip2, libX11, libXext, libXt, fontconfig, freetype, ghostscript, libjpeg, lcms2, openexr, libpng, lib\
rsvg, libtiff, libxml2, openjpeg, libwebp }:

let
  cfg = {
    version = "6.9.3-10";
    sha256 = "e33f021c879f31703f9e620f578ccf7d221a34941589da4bbe967b16a814336a";
  };
in
stdenv.mkDerivation rec {
  name = "imagemagickold";
  buildInputs = [ python pkgconfig zlib.static myglib flex bison glibc.static zlib fontconfig freetype ghostscript libpng libtiff libxml2 ];
  inherit (cfg) version;
  src = fetchurl {
    urls = [
      "mirror://imagemagick/releases/ImageMagick-${version}.tar.xz"
      # the original source above removes tarballs quickly                                                                                                                                                  
      "http://distfiles.macports.org/ImageMagick/ImageMagick-${version}.tar.xz"
      "https://bintray.com/homebrew/mirror/download_file?file_path=imagemagick-${version}.tar.xz"
    ];
    sha256 = "e33f021c879f31703f9e620f578ccf7d221a34941589da4bbe967b16a814336a";
  };
}

yourscript.sh

#!/bin/sh

if [ "$version" == "2.0.0" ]; then
  base=@imagemagick1@
else
  base=@imagemagick2@
fi

$base/bin/convert --version