@MikeH: Yep, if you share a neutral between two circuits that can supply 20A each then the neutral has to be large enough to carry 40A (unless it’s a balanced three phase system with no neutral current such as you would find on an industrial site)
And as you say, while the bus bar is rated for much more than 40A, the neutral conductor running through the house may not be.
@jamesdhatch: a shared neutral definitely is dangerous. Unshared neutrals are also dangerous, for the same reason, but it’s much easier to be safe when you are working on neutrals fed from only one point. If you feed neutrals from multiple points, it’s very easy for an unwary electrician to accidentally shock or kill themselves.
If you have a single phase conductor and a single neutral conductor (as in any normal circuit), and you break that neutral either by removing and separating the looped wires going in and out of an outlet or by disconnecting it from the neutral bus, then that neutral will become live. You are holding a wire that goes back to your outlets, through some appliances, and back to a live bus bar. It isn’t held at neutral potential anymore because you have disconnected it from the neutral.
If you use a single neutral for more than one circuit, you could turn off the isolator for the circuit you are working on, disconnect the neutral, and then find that it was still carrying current supplied by the other isolator and receive a shock.
If you wanted to share a neutral between circuits, you would need to mechanically interlock the overloads so that if one circuit was isolated, they all must be isolated. Then you are effectively back to the same scenario as earlier with only a single circuit
In theory, you can work safely no matter how many circuits share a neutral. The problem is that it needs to be designed in a way that makes it impossible to not realise that the neutral is shared, or in practice somebody is going to end up dead.
Sorry, that ended up being a long response