Is it possible to control a DC motor with Microstepping driver?
To answer your question, yes you can run two DC motors of A4988 Module because the output stages that are driving a bipolar stepper winding are H-bridges and you need two of them (Check here for more on bipolar motors) .

Controlling two motors with different speeds is out of mind. Because there's no different pins.

Now to answer that how would you control the speed. You need to pulse step pin.
You might need to set the micro-stepping modes. Like 8/16 micro-step per step.
Check the current graphs. Because you are approximating sinusoidal waveform. 
Motor might start at some value of current(which can be controlled by pot as you mentioned) 

EDIT : Quick Google search gives me this link
Good Resource. Let me bookmark.