I have 3 waste tanks
Black Grey and a Galley.
All three drain thru same main drain line.
I permanently installed another drain valve on the common drain line.
With it closed I can shuttle waste water (it's all waste water and grey can smell just as bad if not worst then black) between tanks by opening the 3 individual tank valves.
I have 138 gallons of waste tank capacity :B
And I have never had a problem with any of the tanks/drain lines/blockage/pluggage etc.
I remember a time not so long ago that RV's only had ONE waste tank
Added benefit of the extra drain valve...no surprises when I remove drain cap :B