Interesting article on co-axial efficiency vs. flat. I've heard arguments for and against putting smaller props on the bottom with different pitch. This essay and video explains it thoroughly.
I am currently building an X8 co-axial and am using that article and video as a guide before choosing props to test the theory. Also going to try it on my Y6.
I am converting my 3DRobotics Y6 with APM 2.6 now to a Talon 650 X8 with Pixhawk.
I wanted to read more before I invested in motors and props, so for now I intend on using my existing motors and props.
It will at least allow me to see if I have any increase in flight time with the new frame, lighter camera and more efficient wiring.