From this angle, it looks like you have 4 X-axis toolhead rollers that are applying pressure to both rails possibly squeezing them together. This will certainly cause the rails to bend in the center unless there are actually 6 rollers there (2 sets of three triangulating on each rail) and the other two just aren't visible from this angle. You want to triangulate on each rail like the Y-axis sliders. Usually two of the rollers are are fixed and one of the tri-rollers has a cam bolt to adjust the play out of the system.Just about done with the designing!
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