Hello, Pauline.
It should be possible to control two stepper motors with two MC33926 shields and an Arduino. If you want to stack the shields on top of one another, you will have to remap one of the shields; however, it will probably be easier to just use the pins on the left side of one of the shields and make connections to the unused pins on the Arduino.
For controlling a stepper motor, you might find the code in this post helpful.
Another user has posted a proposed wiring diagram for connecting three VNH5019 shields (which are very similar to the MC33926 shield) to a single Arduino Mega that you might also find helpful in this thread.
- Jeremy