( Not too happy about downloading a video file - would preer a YouTube or Vimeo link )
I'd prefer some better names rather than "stepper" and "mystepper" - Iike Xmove and Ymove or Swingarm and Lifter
The motors are doing what you ask them to: You move "stepper" a bot forward a bit backward. Then you move "mystepper".
The problem with the Stepper library is that the calls block - ie. no other code is executing while one motor moves. So the only way to get simultaneous movement is to call the two stepper motion with only one step, and the relative number of times you call them is the ratio you want the motors to move at.