Well that's my first rally done with the Op Tech dual harness and I'm very happy, had a 1Ds with 24-105 and Sunpak PF30x on the left side, a 1D Mk2 with 70-200 F4IS and 430 exII on the other side, used them for up to 3 hours at a time without any fatigue, the cameras are right where I need them (once I'd set up the straps to be the right length) and the whole rig seems very secure.
The harness even fits over enough coats required to keep the wind coming off the Irish Sea at Blackpool out.
I also got an Op Tech pro strap which uses the same camera end connectors so if I only want one body at a time on a neck strap it is an easy switch.