The common ground I was refering to is a common ground between the receiver side and the battery side in the ESC. In the fully isolated CC ICEHV 80, there is no connection between the receiver and battery sides of the ESC for either power or ground. This allows you to take any two cells from your 10s pack and feed them to a regulator and use a second regulator driven by a small lipo for backup set to a lower voltage.
If you have a ESC without that ground isolation , such as a CC ICE 100 with a BEC, then using the motor battery is possible but you must use the lowest two cells in the overall pack(8-10S) This is not a very good idea when you have two balance connectors and other ways to screw up.
Also,there is no good way to use two regulators on the same motor battery without making smoke nor should you want to do that. How many motor batteries have you seen ejected? Happened at the Nats this year.
Another thing- you cannot parallel switching regulators such as the SmartFly this way unless you run a BattShare. Of course that puts you back to a single point of failure. Stick to Jim or Ed's regulators.

Has anyone checked the ripple voltage at the balance plug and at the regulator output? I haven't noticed any issues and my servos have no buzz. I have over 200 flights on a system using a CC HVICE 80, a 350 2S lipo, 2 tech-Aero regulators with separate switches and a connection to the motor battery for the second regulator on the lowest two cells of a balance connector. I feel the two separate switches are very important. I check both lanes for independant operation before each flight. That is difficult to do with a single switch or plugging in connectors. The 350 Lipo will last for weeks without dropping off enough to recharge.

John