Controlling multiple stepper motors non-simultaneously
Like @chris! suggested, you could include two Stepper variables. So in the line where you were initializing the variable, you could instead have:

Stepper steppermotor1(stepsPerRevolution, 8, 10, 9, 11);

Stepper steppermotor2(stepsPerRevolution, 4, 5, 6, 7);


and then continue on with the code for speed, rotation and so on for each motor.