Having the same nixpkgs sources for Nixos and macOS cannot be guaranteed, because the the branches are independent from one another and there’s no target in hydra that performs both the NixOS tests and darwin tests, from what I understand, but I may be wrong.
I’ve asked this here but it went unanswered.
In the end, I use two sub-flakes, one for darwin and the other for Linux that reference the main repo where most of the outputs are defined. In this way I can “override” some of the main flake outputs just for one of the OSes, while keeping most of the stuff declared in a common “base” flake.