Hi custerhank,
Your situation is identical to ours. We have a 23 gallon gray tank and a 35 gallon black tank. The tanks are in fixed locations, tank assignment is determined by the location of the toilet within each floor plan.
When we picked up our PC-2350 from the factory back in 2007, Phoenix explained to blend the tanks because the gray tank will fill before you run out of fresh water. You open the gray valve first, wait 5 seconds, then open the black tank valve, wait until the water slushing sound quiets down, then close both valves. The process floods the near empty black tank with gray tank water. This is a very good thing because the gray soapy water helps clean out the pee & poo. Unfortunately it is a hassle to go out to do that at the worst opportune time....when taking a shower. We got good at anticipating it so we are proactive.
One day I hope to wire in redundant tank control switches mounted inside the bathroom vanity (out of sight) so I can flood the black tank from inside the bathroom. Our bathroom vanity is directly above the sanicon system so wiring them should be very simple. I have the switches but lack the motivation.