It is NOT permitted to share a neutral in any other situation. If you were to share a neutral with two breakers on the same leg of a panel, both circuits could draw the breaker limit (lets say 15A) making the shared neutral as much as 30A return current!

Can you use a common neutral from another circuit?

Neutral conductors shall not be used for more than one branch circuit ... Multiwire branch circuits are an exception. Correct. All neutrals in a box must be separated by circuit, and all ground wires must be connected together regardless of circuit.

Can two 20 amp circuits share a neutral?

One contractor mentioned running a single 12/3 wire from main panel to the bathroom and using one one hot wire for 20A circuit A, and the other hot for 20A circuit B....but would one 12/3 work, be safe, etc.? Yes it's okay. It's called a multiwire branch circuit, and when done according to the rules, is perfectly safe.