Light Grey: Y-axis rails, four times cost but they are a structural item and of minimum size.
Are you bolting through the top flange into the rail? it will not be that structural as the top and bottom sections of the C channel can flex. there is no fixity between the beams (as the bearings rotate) so they wont combine as one structure, each beam will work separately but split the load between them. the torque from the cutter will rotate the y axis beam so one will bend up and the other down.

What size? the really small open bearings have fewer rows of ball bearings so have a poorer radial load distribution, you really need 4 or 5 rows.

The orange plate should prevent almost all torque twisting the blue C-sections.
Unless I'm looking at it wrong the orange plate is the Z axis carriage and if so that will be what is causing the beams to twist!