What you miss with this gantry design is that the rails must be as low as possible, at least one of them. Otherwise is pointless. Now you are just making a big lever.
As it is now is weaker than normal gantry.
The big+++ of this design is eliminating overhang and stiffening at the bearing blocks and lowering the assembly and eliminate gantry twist.
But as i see it you just lengthened the lever and didn't eliminate twist. Ok, square z casing will be stronger but will push more the gantry beams and bearings.
Ideally 3 rails at the gantry will be perfect as i see it with this design.
But adding third couple of bearing blocks on a normal gantry Z will do the same, its easier and cheaper. And 2 beams soldered together will be stronger than 2 separate beams as is your case.
So, try to lower one of the rails.

The point being here is that this design needs to be done correctly cause it will allow high speed machining or deeper dig, but you will need as i see it at least 3kw spindle, or it will become the weak point. As Dean said before.