First step, go to VDH forum and read in there. I believe there is a sticky on doublehose 101.From what I've read thus far looks like it's sitting too high?
you do want the tank as low as you can get on that back plate... perfect world is cans centered in between shoulder blades (top of can below base of divers neck).
A couple things I've found... positioning is as huge as they say at VDH. When you roll on your back, it will chipmunk your cheeks out, but won't "force an inhale".... your mouth is just the high point of the loop, but your lungs are still right near the cans. Practice clearing... It's not difficult, just a different process than a single hose. When you roll left during an exhale to clear the exhaust hose, you don't have to roll all the way over.
Bear in mind I've only used my DH a few times (2 dives, bunch of short pool sessions)... most of this is gleaned from the VDH forum.
Respectfully,
James