hello to everyone! I state that I'm just a beginner in programming of 'arduino and I'm working on a project that I would like to present at the final exams but I need the' help of someone more experienced because, as much as I informed I can't undestand what i've got to do ... the project consists essentially of two stepper motors whose movement is controlled by a subspecies of encoder i've created myself that would allow a much higher resolution than the stepper motor itself. for constructive questions I need to control two motors with speed and direction completely independent of each other (as I assumed the motor nema-23 with absorption of 2A and voltage of 4.5 V and 200 steps/rev) and I have to control it using the Mega2560 rev3 and one / two shield dedicated to the motors to simplify the construction of the whole, in practice I have to control the speed of the motor and change it if necessary every tenth of a second: someone can tell me which shield wold be better for this application? thanks for helping me! 
I did a Google search for 'shield dual stepper arduino' and the top result is:
Dual Stepper Motor Driver Shield for Arduino - RobotShop
http://www.robotshop.com/en/dual-stepper-motor-driver-shield-arduino.html
Shortly after that comes:
Arduino Dual Stepper Motor Driver Shield - ITEAD Studio
http://imall.iteadstudio.com/im120417015.html
And on Amazon:
Amazon.com: Dual Stepper Motor Driver Shield for Arduino ...
http://www.amazon.com/Stepper-Motor-Driver-Shield-Arduino/dp/B006YM2VBE
These all look identical so pick the one that is cheapest.
can they drive bipolar stepper motors?
the project consists essentially of two stepper motors whose movement is controlled by a subspecies of encoder i've created myself that would allow a much higher resolution than the stepper motor itself
What is this ? Are you just making this stuff up as you go along ? What sub-species encoder ? Speak Engineering . Absolute , Quadrature,
Incremental ? Don't just make something up and say I'm going to use this thing I am going to invent .. That's non-sense. State what kind of encoder you want to use.
it' s not properly an encoder, i've invented it 
however i just need a shield to control those motors... how to control the encoder is not a problem
SEE Reply#1
You don't always need a shield to control two stepper motors.
See below:
it would be perfect,it' the same thing i have to do but wich driver did he use?
marcogirardini:
it would be perfect,it' the same thing i have to do but wich driver did he use?
http://www.bps1025.com/watch.php?vn=Minimal+Arduino+(ATmega8+driving+a+Stepper+Motor),-AjjbSN02n60.html
"The darlington array ULN2003A was used to drive the four coils of an unipolar stepper motor."