foo.nix

let
  pkgs = import <nixpkgs> {
    overlays = [

      (final: prev: {
        haskellPackages = prev.haskellPackages.extend (self: super: {
            test-notexist-a = self.hello;
        });
      })

      # PROBLEM: a second overlay adds test-notexist-b but causes the
      # previously added test-notexist-a to disappear.

      # SOLUTION: use the override style which gets the old override attrs (??)
      # and then manually generate the overrides attrs (??) by calling
      # old.overrides and then combine it with the new one
      (final: prev: {
        haskellPackages = prev.haskellPackages.extend (self: super: {
            test-notexist-b = self.hello;
        });
      })
    ];
  };
in
{
  hello = pkgs.haskellPackages.hello;
  a = pkgs.haskellPackages.test-notexist-a;
  b = pkgs.haskellPackages.test-notexist-b;
}