newScope.nix

# a.nix
{ newScope }:
newScope { a = "a"; } ./b.nix { }

# b.nix
{ newScope }:
newScope { b = "b"; } ./c.nix { }

# c.nix
{ a, b }: a + b

# default.nix
with import <nixpkgs> { };
callPackage ./c.nix { }