According to your diagram - you will only have two coils per motor energized at any one time. As each coil is rated at 3.5 Amps max then for 2 stepper motors the power supply needs to be able to supply (4 x 3.5) 14 Amps. (The most common stepper voltage used for small machines seems to be 24 Volts). The formula for calculating the series resistance is given in the driver spec.
Also, as it says in the driver spec, at this current the board is going to get hot so forced air cooling may be necessary.
Tweakie.