I've been told that the linear bearings and rails are stiffer than anything you could bolt them too.

If that is the case then you'd be better off combining all beams in the gantry in to one structure.
I think you now support the Z on two sides for stiffness, but you actually lose torsional stiffness this way.

Also, it will be extremely hard to line up all parts to run smoothly, with multiple carriages on multiple carriers.
All in all, though it looks very nice, I feel it is overly complex but will under perform.