At this risk of sounding a bit overbearing - have you tried to understand what is happening with a "blocking" call? My explanation may not have been clear, sorry about that.
Try and use the "#" button on the input editor so your code comes in a box, ie is surrounded by the "code" brackets. (it is not just pretty that way, there are some formatting issues, too)
Meanwhile, the problem is that you still use
stepper.step(200);which does 200 steps, and while doing so does NOTHING ELSE. SO you have to "manually" in your program alternate between one step with one motor and one step with the other motor in an apprioate ratio
stepper.step(1);