We have just one holding tank for black and grey (189 gallons)... the fresh water tank is also 189 gallons. We have a 'grey water bypass' feature if needed. The bypass has a hose connection so we could water trees/bushes/flowers if needed.
There is virtually no risk for the grey/black tank to back up as there is an alarm on the system. A lady's voice will come on saying something like 'holding tank is approaching full' and will not stop until you hit the 'disarm' button. The same lady (I've looked for her but she hides well).... will also say 'fresh water tank approaching empty'.
Personally, I like the one holding tank system as you can get more liquid in before it needs to be released vs. having two separate tanks.
MM.