Mixed channel NixOS derivations

gistfile1.txt

# Have different channels added
$ nix-channel --list
beta https://nixos.org/channels/nixos-16.09
nixos https://nixos.org/channels/nixos-unstable

# Install derivation(s) for current user
$ nix-env -qaP | grep pirate-get
beta.python35Packages.pirate-get                                python3.5-pirate-get-0.2.8
nixos.python35Packages.pirate-get                               python3.5-pirate-get-0.2.9

# Install derivation(s) globally in configuration.nix
{ config, pkgs, ... }:
let
  unstable = import <nixos-unstable> {};
in {
  environment.systemPackages = [ unstable.packagehere ];
}