Connecting two Motor Shield R3 at Arduino Uno R3

Using Arduino Motors, Mechanics, Power and CNC
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Purchased an Arduino Uno R3 and two Motor Shield R3 (http://arduino.cc/en/Main/ArduinoMotorShieldR3).

Running one stepper at one Motor Shield R3 is no problem.

But it is (and how) possible to connect the SECOND Motor Shield R3 at Arduino to control TWO steppers with ONE Arduino?

No problem to buy an Arduino Mega 2560 R3 if it is necessary but I need a x/y-control. Only one direction is not enough :frowning:

Thanks
Andreas