connecting grey tanks to get more capacity for the shower

A cheap way to accomplish the same thing is to put a valve on the sewer pipe outlet. When the shower tank gets high close the outlet valve and open both gray tank valves. They should balance out giving you more shower capacity. They you can close the gray tank valves.
We do this when necessary and it works out well.
Of course, another way is to chose a site with sewers...

Guess I should have read the previous posts for the same idea. Great minds....