? Concerning new residential wiring for general outlets and lights. Does the NEC allow the use of 2 conductor wire say 12/3 to power 2 individual 20 amp circuits, where they share a neutral and a ground, and do the circuits need to be supplied by different 120v legs? Thanks BL

Shared neutrals are allowed, called a multiwire circuit. There are a couple of issues to be aware of.

If the two circuits feed one duplex, split with both circuits, the circuit breaker needs to be a two pole breaker.

The neutral must be spliced at every device and not just rely on screw terminals for continuity.

If the circuits feed critical equipment, there is a chance of damage to the equipment, if the neutral breaks higher than normal voltages can be sent to most if not all the equipment. If only a lighting circuit, then all that is lost are some light bulbs. But the initial cost of running two wire circuits to important equipment is worth the peace of mind. There is a risk with shared neutrals, but depends on the quality of the connections.

Yes if two circuits share a neutral, you must use one circuit on leg A and the other on Leg B of a single phase panel. This is done to insure the current in the neutral will be "balanced". Two circuits drawing 20 amp will have zero amps in the neutral, and less as the current in each leg goes down, if not on oppsing legs, then the neutral will carry 40 amps, on #12 wire.

But here you can see you will be applying 240 volts across both hot legs, and imagine the neutral is broken, the current flow in each can seek out other devices and flow through those device seeking a return, causing really weird voltages imposed depending on the impedance of both circuits.

The choice is up to you.