I thought about horizontal dual-X gantries on mine, but I didn't want to buy four more linear rails for a proper central-column system (though it would be much better for 5 axis with any degrees of freedom in the head). I like the legit bridge-mill dual-Z as well!
Hey awesome build! I've just dug into DIY printing machines for prototyping a new product. This unit looks impressive for the Z axis range and dual gantries. Following!