i have a problem with box flex - with a span of wood that long without any supports it will flex- loosing sound engery... shared air volume not a problem if box regitity is built into it, a few supports (one between subs) and corners, make a much stiffer box, i am now in the habbit of doubling up on the mounting surface where the subs mount (or at least doing a ring so you have 1 1/2 of wood to screw to (i also now use t nuts and allen head screws to hold subs in) these few things make a huge difference in durablitly of the box. plus it allows you to give the box a more custom look.