Running two different nixpkgs/NixOS releases on one machine, possible with specialisations?

I have a flake based NixOS setup where my main system tracks a newer nixpkgs revision (currently on unstable). I also want to run an NVIDIA vGPU stack that only works correctly on an older nixpkgs release, specifically NixOS 25.05. What I initially wanted was to use a specialisation for this, something like: specialisation.vGPU.configuration = { ... }; The idea being I could boot into a “vGPU” variant of my system that uses the older nixpkgs release just for the vGPU stack, while my normal...