sharing gray tanks

Yes. Maybe. If you have one single dump outlet, you can add a blade valve right at the outlet. Then leave your two tank valves open, and the level will equalize between the two. If you have two separate dump outlets, there won't be a clean and easy way to plumb them together outside the coach.

Many of us have added that blade valve at the dump connector. I use it to back wash my black tank using gray water at the dump station. It is the same concept, which is to let the tank levels equalize.