Can 12 2 w gnd wire be used for two different 20 amp circuits safely meaning I would have two separate circuits on two separate 20 amp breakers There would only be 1 white wire returning?

Yes, you can do this, but there are a few rules to follow. The white wire(neutral) should be solidly made at all device boxes in other words do not use the receptacle terminals, make all the joints solid and pigtail to the receptacle. The breakers need to be adjacent to each other in the panel, side by side on opposite buss bars. If you put them on the same buss you will overload the neutral. Use a handle tie on the breakers.

I am sorry. It was the wrong question. The question really should have been: Can you wire two separate 20 amp circuits using one single romex 12-3 w/ground. Each circuit would have it's own 20 amp breaker. The Red wire would be landed on one breaker. The Black wire would be landed on another breaker. Only one #12 white wire would be available for neutral. My theory is: If you were to load both circuits to say 19 amps each, you would have 38 amps being used. The problem lies in the single white neutral's ability to handle 38 amps on it's own. Wouldn't this be a problem? Would this configuration pass in spection? You can do this with 12/3, you still have to make up your neutrals at device boxes. the neutral would not have to handle 38 amps as long as you use opposite 110volt buss. the current on the neutral if both breakers were drawing 16 amps each would be "0". The neutral only has to carry the imbalance of the two circuits. If one breaker draws 16 amps and the other 8 the current on the neutral would be 8 amps. Breakers have to be on opposite buss! for this to work. Yes, it should pass inspection.