Open up marlin in the arduino software. Go to the Configuration_adv.h tab. Look for #define Z_DUAL_STEPPER_DRIVERS. Remove the // in front of it to turn it on. assuming your endstops are also split into their own connections, uncomment #define Z_DUAL_ENDSTOPS too. Make sure you set the #define Z2_USE_ENDSTOP _ZMAX to the right connector for the Z2 endstop. To adjust the offset, send "M666 Z-" then your offset. (x: M666 Z-0.40). Save it to your EEPROM once you get it dialed in with M500.