Most of the codebases of both iOS and macOS are fundamentally the same kind of (low-level but architecture-independent C and Objective-C) code, and encouraging team unification will make that even more true—if the same people are working on both AppKit and UIKit, for example, it's a no-brainer to unify them rather than maintaining that duplication.
The exception is the kernels and drivers of each OS, which have to consider things like architecture, but I would expect that it'll still be the same individual engineers working on these who always have, no matter what their team structure looks like now. Whether or not Apple has a formal "macOS kernel team" or "iOS kernel team", those groups definitely exist and have rather static membership.
(In fact, one main benefit you might see from this is that Apple has hardware hires—many from Intel[1]—who have stepped up to apply their knowledge of formal verification methods to the iOS kernel and drivers, but haven't necessarily had the flexibility to spread that love to macOS. That can change now.)
[1] https://danluu.com/cpu-bugs/ (search for "Apple")