This method is described on Russell Brown's website, among others. I've never found out what this method is called. I'll just call it "hue / saturation layer" method for lack of a better term.
Some prefer the hue layer method because it overcomes the two drawbacks of the channel mixer method: lack of fine control and the inability to hold the brightness steady. Some find the channel mixer easier to predict and control.
After creating the two hue/saturation layers as described this is what the photograph looks like:
For this photograph let's turn the sky black and the building light gray. Open the first hue/saturation layer. After playing with the hue slider for a bit you'll find that -140 creates a nice dark shade of gray.
But the sky still isn't quite dark enough. Let's try adjusting blue. Changing the hue doesn't seem to do very much. but changing the saturation to +50 seems to darken the sky further. The same for blue.
Here's the final result: