"If I go with option A wouldn't that effectively make it 320 CFM?"
Yes it could. If they are vented correctly. Meaning the vent is sized correctly, not to long, not to many elbows, ect... Some people have the misconception that you can pipe both hoods into one 6" pipe then run that pipe 40 feet acrossed their basement up and around a half dozen water pipes and wires that happen to be in the way and think you still have 320 cfm. It dont work that way. You may get a duct sizing chart with the hood, or ask on the board here. There are a couple of us who have done that for a living. One of us could probably size you duct for you with a bit of info.