Stepper.h

hi,
I’m using this simple code to drive two stepper motors.
the motors are also the wheels of a little robot.
So I want to move both wheels simultaneously, but this code moves the first one and then the other.
Where’s the problem?

thanks in advance

#include <Stepper.h>

#define STEPS 48

Stepper stepper1(STEPS, 12, 11, 10, 9);
Stepper stepper2(STEPS, 8, 7, 6, 5);

void setup()
{
  stepper1.setSpeed(20);
  stepper2.setSpeed(20);
}

void loop()
{
  stepper1.step(100);
  stepper2.step(-100);
}

try this (step is blocking):

void loop() {
  stepper1.step(1);
  stepper2.step(-1);
}