That is indeed a feature of the Galil.  It's called Dual Loop and uses both encoder inputs to eliminate backlash.  However, it is not supported directly by the Galil plugin for Mach, meaning that there is no special configuration for it in the plugin config.

The Galil commands to implement it are CE and DV.  They are described on page 115 of the 40x0 user's manual.  Also, there is a tutorial video on Galil's website that explains Dual Loop and it's tuning.  It requires different KP, KI, and KD parameters than just an axis with one encoder.

The plugin does not change the values of KP, KI, KD, CE, or DV.  It assumes that these values are set and "burned" to the controller's NVRAM.  So you can configure dual loop with KP, KI, KD, CE, and DV and burn the values into NVRAM using the BN command.

Steve