one stepper motor is 1.7A and DRV8825 Current Limit Vref is about **0.75V**, but if two stepper motor and then need 1.5A output for two of them?

No…If you wire the two motors in series then the current flowing from the driver flows through both motors. As there are now two sets of two coils the stepper driver has twice the coil resistance to overcome to push 1.7Amps through the pairs of coils so automagically adjusts the *voltage* at it’s terminals to push that current through the two coils.

Ohm’s law states V=IR

As each motors coil is about 2.4ohms and you want 1.7amps flowing through it then you need 3.78volts to do it, if you double the resistance (by adding another coil in series) then you will need twice the voltage to push it 'ie (2.4x2)x1.7=7.56v. Now as you are supplying the drivers with at least 12v you have the headroom to run the motors in series…things start getting a bit tricky if you are using motors that have a maximum current of 2 Amps and a coil resistance of 3.5ohms because you need at least 14v to push that current around the coils so have to use a higher voltage power supply…usually 24v.

If you wire the motors in parallel then you would need 3.4Amps coming from the driver as the current is split between the coils and each coil would want approx half each.