Combining black and Gray

With my tank configuration it's not a problem for me to push some gray water to the black tank. The bottom of my gray tank is at about the 1/2 to 2/3 level of my black while the top of both tanks at the same level (underside of coach floor). So I can easily push gray to black without over filling the black. I use a third valve as Mark posted. I don't normally comingle. With all valves closed I'll open the gray then black. Count to five and shut the black. I want to keep gray flowing into the black rather than allow the flow to settle. Plus I want gray only in the gray to use as a post black dump flush of the hose.

I also do this when dumping and I can't use the in tank flush system. Send a slug of gray into the black to help remove any remaining solids.