I have some experience with steppers, bipolar only and unfortunately not with the l293 nor with Tom Igoes library found in the standard examples.
If it works a bit like most other libraries this may be… an example to drive 2 motors.
// steps value is 360 / degree angle of motor
#define STEPS 200
// create a stepper object on pins 4, 5, 6 and 7
Stepper stepper1(STEPS, 4, 5, 6, 7);
Stepper stepper2(STEPS, 8, 9, 10, 11);
In Toms example code he didn’t use delays, not using them is often the best…, but I just adapted your code a little, left the delays in the code and besides controlling the steppers it should just wait for a small moment after each turn.
Hope this works, if you don’t have 2 steppers yet you could run the example with 1, take all power of the board and circuits (!) after the first test, connect the circuitry to the arduino pins of the second motor and… power the whole circuit again.
By the way… I do guess this example won’t give you steppers operating really simultaneously, because of the stepper.step() function. In the example it uses 200 and -50 steps (whole and quarter turn), while 1 motor is turning I expect the other to be motionless.
It is possible to drive both at the same time by sending the right pulses to those 8 pins, but you’ll need to check out how this works with your steppers and probably need to write routines to adjust speed.