You can't run two for loops at the same time. You need one loop that makes the motors keep stepping until the complete motion has been completed. Keep in mind that only one motor can step at a time. There are plenty of examples around for driving two stepper motors simultaneously (involving making each complete only a tiny fraction of the overall motion on each loop iteration). Time (for you) to do some searching.
Perhaps start with looking for Grumpy Mike's web site. I know he's posted code to drive two motors at once.