James,
I don't know if there is any regulation in place, but there are a few reasons to keep them separate. If you notice your gray water tank is getting nearly full, you can cut back on things like long showers and switch to washcloth or sponge baths, and cook things other than 10 gallons of pasta at a time until it's convenient to dump. You don't have that option with the black tank; GIGO. The gray water is also used to flush the hose after dumping the black tank. More sanitary that way. In theory (if not in practice), you could use the gray water to water outdoor flowers. However, gray water tends to be as stinky as the other stuff.