Phew! That's another code difference between the US and most European countries yet unknown to me.
Here there's a somple rule: one hot, one neutral, all the way up to the panel the circuit is originating from. Circuits that are on different breakers MUST NOT share a neutral. Only exception: split circuits taken from one 3ph circuit may have a shared neutral and are not even required to be on a 3 pole breaker as long as there is a 3 pole switch somewhere upstream.
Generally that practice is extremely frowned upon though as people say with small wires like used in household wiring broken neutrals are far too likely. So hardly any multiwire circuits here.
