[folivier]

Nope, you should have only 1 waste tank which holds black and gray waste. What's discussed above are the valves that allow you to direct the gray water out either on the ground (if permitted) or into the sewer system while allowing only black waste to fill the tank. When we had our Newell we would sometimes open the bypass when washing clothes so the tank wouldn't fill as quickly.
Hope you can come to the rally in Spearfish next year.