"John Rumm" wrote in message
o.uk...
dennis@home wrote:

Not really - even wiring to the 17th edition using the "standard"
arrangement with 3&E run between floors to facilitate the "other" two
way switch, you will still have a switch position containing live wires
from two separate circuits.

Why?
You have live running to the top switch, two switched lives from the top
switch to the bottom switch and one switched live running back to the
fitting, only one live circuit is needed and no neutrals run down the
cable at all. Operating the switches will either turn the light on and
off if the wrong circuit is isolated or do nothing.

And that means the bottom two gang light switch has lives from the
upstairs and downstairs circuits. (and the upstairs likewise if two way
switching is implemented in both directions).

If its a two gang then the lives can come from any circuit, there isn't a
rule that says otherwise.