EDIT: I read OPs post again and my post pretty much has no use lol....sorry...
You can somewhat cross connect them very simply. It’s the same method boondockers and full timers use to flush the black tanks using the grey water instead of fresh water wasting.
This only works if you first dump the black tank (when using this method to flush) or the black tank is barely full and your grey tank is close to full (used to extend stay/dumping)
#1- install a Valterra twist on valve to your sewage dump port
#2- make sure the Valterra valve is closed and open your grey tank valve, this will allow the grey water to fill the dump pipes but keeps it from dumping out because your new external valve is still closed
#3- now open your black tank valve while still leaving the grey open too and the grey water in your lines and tank will rush into the black tank to equalize the pressure
#4- the black and grey valves are then closed after you’ve let the fluids equalize. You won’t get black water crossover because the grey water was already in the way and rushed into the black tank. The soapy grey water will help flush out the black tank when you do dump it later.
I do this in short intervals after I dump my black and it’s empty. Since we all know to dump black first anyway there is still a full grey tank. I’ll use same procedure and let grey water rush into the empty black for a minute or so then close grey and dump black. After a few of these grey water flushes you should be almost empty on grey and helped flush out your black (win, win).
You can only do this using the external added valve, otherwise you’d open your dump pipe cap and get a nasty surprise!