When I'm running a chest rig I either use a Camelback MULE or I stuff a few water bottles into my GP pouch.
Anyways, this thread needs pics.
MAV 2 piece
MAV 2 piece with X back
I never load my MAV with enough weight in airsoft to really need the x-back. But with 6+ AR15 mags, 2 pistol mags, and some water bottles, its heavy enough to benefit from the x-back.