adrian_k wrote:...on a 'normal' balanced TRS I would connect screen to the sleeve and the other two to tip and ring paying attention to getting them the right way round.
But each individually jacketed core in this two way multicore is a three wire, one of which is not insulated so I would treat as an 'earth'...
Yes. That un-insulated one is the drain wire and that connects to the plug sleeve so that it is grounded.
I'm then still left with this very flimsy foil screen that encases both cores.
If each individually jacketed cable is already screened -- and it sounds like it is -- then this flimsy screen is probably just part of the overall packing/wrapping. You can't solder foil screens directly, so unless there's a drain wire in there I'd leave it alone!
And even if there is a drain wire, it would necessarily be outside the individually jacketed cables going to each plug... so how on earth are you supposed to connect it to the plug neatly?
This sounds like a secondary screen for use in hostile environments when the cable is connecting stage boxes or similar.
Can you provide a link for the cable supplier so we can see exactly what you are using, please?