I don't know why anyone would care if the black tank back washes into the grey tank. Pretty much every residence I've ever seen has a single sewer system for both black and grey, and some high end coach conversions also use a single large tank for both. I've used the third valve setup on our RV's for the past dozen or so years, and besides using it for flushing as described, I also use it combine the two tanks at times to extend the grey tank capacity using extra black tank capacity. Works great for extending boondocking time between dumps.
