The way you are initializing the steppers in your
initSteppers() function in the code you provided appears to be correct. There is a note on the Arduino reference page you linked to that when you use the
SPI.begin(int ssPin) function with a slave select pin, that pin becomes unavailable from the
digitalWrite() function we use in our library, so you should not use that function for the two pins you are using with our board, but it should OK to use the function with other SPI SS pins.
I was able to get two of our drivers working with a Due using the code below. You might try a very simple test case with just two of our drivers to see if you can get that working and then add some simple code to see if you can get the graphics driver to respond at the same time. Is there a simple example sketch for the display you are using? Also, using a logic analyzer (if you have access to one) to look at the SPI pins might help with your debugging.
AMIS30543 stepper1, stepper2;
SPI.begin(); //Do not use the CS pin numbers of any AMIS driver boards.
//SPI.begin(11); //It's OK to call SPI.begin with a different pin number if you have another device that uses that pin.
//SPI.begin(4); //If this line is uncommented, the AMIS driver using pin 4 will no longer work
stepper1.init(4); //Slave select pin for one AMIS driver
stepper2.init(10); //Slave select pin for the other AMIS driver
// Set the NXT/STEP pins to outputs
pinMode(12, OUTPUT); //Connected to the step pin of one AMIS driver
pinMode(2, OUTPUT); //Connected to the step pin of the other AMIS driver
// Give the driver some time to power up.
// Reset the driver to its default settings.
// Set the current limit. You should change the number here to
// an appropriate value for your particular system.
// Set the number of microsteps that correspond to one full step.
// Enable the motor outputs.
//The readPosition() function pulls data over the SPI bus, so if this
//responds appropriately, then the devices are working
//Step one driver twice and the other one once so the positions do not change in sync
//This is so we can be certain we are reading different values from the two different drivers