connecting grey tanks to get more capacity for the shower

I have 2 grey tanks on my Grand Design Reflection and I put one of the end of the 3" dump line.

Valetrra

This allows me to keep this valve closed and open both grey tank valves to tie them together.

When I dump my tanks close both grey tank valves and hook the stinky slinky to the outlet then open the Valterra valve and then the black and let the black drain and then you have both grey's to empty to rinse the slinky.