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.
Think about it... it's your license. And if something goes wrong, guess who the lawyers point the finger at? And they're not gonna want to hear you've done it that way for years and there never was a problem. It's the potenital they'll nail you on. And if you say there's no possibility of that happening, you'll then be labeled as incompetent.
What is so difficult about running dedicated neutrals, or a correctly sized neutral in the case of conduit? I understand copper is getting expensive, but since when is expense an excuse for shortcuts?
Run a dedicated neutral and this way you'll have nothing to worry about.
Joe
