To clarify: you are using two helical gear racks in parallel but with opposite hand or a helical rack in parallel with a herringbone rack?
In the first case, with opposite hand racks, tightening the screw on the second rack will apply a thrust force in the same direction on your shaft. Play will be eliminated once the shaft loads up the thrust bearing. Gear forces will always act against the thrust bearing so in theory you only need one thrust bearing. In practice your racks will have some pitch error which may not match between the two.
In the second case, a helical plus herringbone rack, rotation in one direction will force the gear to center on the herringbone and rotation in the other direction will move the gear towards the thrust bearing. There will be additional error from pitch differences between racks.
RT