What do you mean by that?
Do you mean you can’t set the drive to twice the Vref of one drive?
You have still not said what current your motor draws.
The driver has a limit to the current it can supply and if you are on that limit then you need to use another driver and simply wire the step and direction pins to the same Arduino outputs. However you have to position the two motors to the same starting position before you attempt to drive them otherwise they will be a permanent offset between the two motor positions.