Layering vocals is a common technique to thicken a vocal part, whether applied to a solo voice or to a massed group of backing vocals. However, there are certain considerations with layered vocals that don’t apply to single vocals, as layered vocals need to have a coherent, solid vibe.
Nice beginning primer. Were there another installment, I imagine these would be covered:
Stereo spread ~ I like to have a fairly wide stereo spread going to the bus; you can always tighten up the bus panning.
Tuning - Depending on the style of the tune, if you're tuning the backing vocals, you'll probably want a 5-10 cent spread on the tuning so they don't sound phasey. You can go 7 or 8 cents below pitch usually, but not quite that far above the pitch with your spread. Typical for me would be tuning one part up 3 cent and the corresponding part down 4-5 cents.