Robin2:
The standard Stepper library is very basic. Try the AccelStepper library. It's run() function can control several motors simultaneously.Be sure to study the examples as its usage is different from the Stepper library,
If you really do want to stay with the standard Stepper library then you will need to make the motors move one step at a time and manage the step timing yourself.
...R
And if you want them to step together, then you will have to interleave the coding for both motors, not do one and then wait and then do the other and wait and expect them to step at the same time.
Paul