I tried this weekend opening both grey tanks to see if they would equalize. I opened both grey water tanks (leaving the black closed) and closed the external gate valve. When the shower and galley tank filled, the second gray tank started to fill too however I had water leaking outside from the underbelly of the rig.
The solution using the external valve doesn't seem to balance the two grey tanks without leaking.
I am going to fill the galley/shower tank again and see if it backs up in the kitchen sink as it did before, without leaking.
Sounds like I'm out of luck trying to use both the grey tanks for the shower/galley??
Works ok if both grays are at the same horizontal level...doesn't look like mine are. Too much pressure to fill the empty tank or another poor spin weld on the shower/galley tank causing a leak at the top. Filling that tank by itself now to see if it leaks when backed up into the kitchen sink.