"I never understood the logic of trying to take this potentially dangerous shortcut. Code or no code, the neutral needs to handle the maximum potential unbalanced current. Period. In the case of two 20A circuits, that's 40A, in which case the neutral needs to be #8 and that's not what you get with 12/4."
If your (2)20A circuits are on opposite phases you would never have more than 20A on the neutral. The neutral will only see the difference between the (2) loads, not the combined total. Circuit A= 15A + circuit B=10A the neutral would carry 5A. Nothing dangerous about it.
