Nix also has a Guix service itself[^1], if you want to do the same in reverse ;)

[^1]: https://search.nixos.org/options?query=services.guix

I love that this goes both ways. What other pair of Linux distros mutually offer first class support for using each other's package managers alongside their own??

It's a cute little testament to the fundamental strengths of the functional package management paradigm, as well as helping users on either side of the fence fill some gaps in a pinch and providing more/easier opportunities to compare notes and inspire improvements.