...each side must be powered independently for that side to work, allowing me to put 18v (via my PH brick) to yard side and 12v to the camera side of the same switch?
Correct.
As a non-electrical guy,
Well, by definition, if you want to play with electric trains, you need to be an electrical guy. I suggest that you find yourself a book on simple electrical principles and components and study them. Try a Goggle search for "toggle switch" for a start.