default.nix

with import <nixpkgs> {};

rec {
  imagemagick1 = pkgs.callPackage ./imagemagick-old.nix {  };
  imagemagick2 = pkgs.callPackage ./imagemagick-new.nix {  };
  both = buildEnv {
    name = "both";
    paths = [ imagemagick1 imagemagick2 ];
    pathsToLink = [ "/bin" ];
    ignoreCollisions = true;
  };
}

gistfile1.txt

[clever@amd-nixos:~/Downloads/d7c269c7ced0c96831af3fbac3a5dc6a]$ ls result/bin//
animate-1.2  compare-1.4    conjure-1.2  convert-1.4  identify-1.2  import-1.4           Magick-config-1.4      MagickCore-config-1.4  mogrify-1.2  montage-1.4  Wand-config-1.2
animate-1.4  composite-1.2  conjure-1.4  display-1.2  identify-1.4  Magick-config-1.2    Magick++-config-1.4    MagickWand-config-1.2  mogrify-1.4  stream-1.2   Wand-config-1.4
compare-1.2  composite-1.4  convert-1.2  display-1.4  import-1.2    Magick++-config-1.2  MagickCore-config-1.2  MagickWand-config-1.4  montage-1.2  stream-1.4