Halcyon do a dual bladder?? Aren't they meant to be only for strokes?
(I did hear of one idea for dual bladders I thought quite creative: Remove the inflator and put a blanking plate over it. Then if you get a puncture whilst on holiday, you can just switch the inflator to the other bladder and not miss out on any diving)
Have you seen the combro backplate? It's what I use, and it's a lot cheaper than the branded ones:
www.backplates.co.uk
As for the harness: A four-meter length of webbing from your local dive store will do as well as anything from the manufacturers.