Split-Toning: namely creating a two-color image where the coloration depends on image luminosity - one color for darker - one for lighter.
I've read of three different ways using Layers and Masks and there may well be more ways of skinning the proverbial cat.
A common element is to convert a base layer into grayscale and then to use it and it's inverse as Masks. But one method then puts a filled color Layer above a grayscale image, another below and a third colorizes the grayscale image itself.
The question remains ... is there a Preferred or "best" Method for Spilt-Toning?
Question prompted by my mentions of Teal and Orange colorizing here.
Ancillary question: are there other ways to skin the cat e.g. not involving Layers?