Like many others here, I couldn't quite work out what Phil was saying he'd found, it make not a lot of sense with the light having two permanent lives, etc. It came to me a bit later though. Its still a borrowed neutral situation, but slightly different to the normal one we encounter.
If you think of a standard conversion method 2way wired in 6243Y. With a switch drop from the landing light to the top switch and then the 6243y out and to the switch at the bottom. Now keep it wired the mostly the same, remove the core linking L1 to L1 on the two switches, which is in with the red permanent live of the switch drop at the top, you are now short of a permanent live to L1 at the bottom switch (but you of course still have one at the top), so that gets linked off the adjacent switch, which happens to be on a different circuit. Result, is it still borrows, (but only half the time), I suppose you could say its more of a borrowed live than a borrowed neutral!
