If you are using the latest Marlin ( RC8 AFAIK ) then there is a setting in configuration_adv.h ( line 268 ff. ) to use two stepper drivers.
This is the safest variant if you don't need the second extruder.
// A single Z stepper driver is usually used to drive 2 stepper motors. // Uncomment this option to use a separate stepper driver for each Z axis motor. // The next unused E driver will be assigned to the second Z stepper. #define Z_DUAL_STEPPER_DRIVERS
This is the safest variant if you don't need the second extruder.