I have done a few projects with the easy driver. I have tried to use the AccelStepper library too, but ended up doing my own controller for various reasons.
First, lets double check your connections. You should have the four motor wires from each motor going to the four motor outputs on each EasyDriver. Your stepper voltage supply coming in should go to M+ on all of the drivers, and the ground from this supply should go to one of the ground pins on each EasyDriver.
Now for the control, each EasyDriver needs a step wire, a direction wire, and a ground wire. The ground should tie from the EasyDriver ground to the Arduino ground to make sure they are tied together. The step and direction lines are the only ones which have to go to the digital pins on the Arduino, and they can go to any of them.
Now in the code you should understand what the line you listed is doing. It is a constructor that makes a new AccelStepper instance. The first parameter sets the mode of operation. In your case this is with an external driver and will always be a 1. The second parameter is the step pin number, so just put in the pin number of the digital pin connected to the step input on the associated EasyDriver. Similarly, the third parameter is the direction pin, so assign it to the pin number you used.