Caterham wrote:

not sure if I've missed something but my guess would be they've used 2 cores as earth to keep earth loop impredance down.

I recall one of my first design jobs was for lighting in a trench for GKN pressings and thats eactly what we did. larger cable into adaptable box for local lighting.

depending on what your dado trunking is intended for perhaps a radial circuit might be easier etc.