Added steel channel to X-axis c-beam

[routercnc]

The split tubes have the advantage that the Y axis can be very close to the gantry as it does not need to be offset to accommodate the ballscrew behind it which usually takes up more space than the rail and carriage.

But it is nowhere near as stiff a shape as a single rectangular gantry. In vertical bending the 2 tubes are only twice as stiff as one of the tubes on its own. But when joined as a single large tube they are 2^3 times stiffer (8 times) for example.
As mentioned by magicniner there are similar benefits for torsion for similar reasons.

That is why when you see 2 tube designs with the screw in the middle the builder often plates them together at the back to try and gain some of this advantage back.

If you go with the single large rectangle then the rails can be spaced out on thick solid bars welded to the section to give the ballscrew clearance. Sure, this puts a moment on the section but you can afford a bit of this loss as you have so much to start with.

Another variation is to have a tall more slender section vertically and have another horizontal section behind in a reverse L shape. This is popular when used with aluminium extrusions and the ballscrew can sit behind the section driven off a good bracket that often houses the Z stepper too then the rails can be tight against the section again.

I’d favour the large rectangle as per your blue drawing but the above gives you some options and reasons.