There should be no reason that a dual drive gantry racks unless one stepper is either not calibrated or is losing steps. Are the motors software slaved or hardware slaved. Each has to have it's own drive card and the only thing they share is signals. Also motor tuning needs to be the same on each axis if they are hardware slaved.
Hopefully they are not just direct drive rack and pinion. The pinion gear performs a step up ratio (rather than reduction) which gains you lots of speed but really poor resolution. Each step is about .015 or more. Both sides should be out no more than two steps (.030) even at that.
Tell us a little more about the system and mechanics and we can help you track down your problems.
Making the gantry more rigid will just force the motors to fight each other more. You need to resolve the out-of-sync issues or lost steps that is causing it.