You need to completely reorganize your code so that both motors can make a step in every iteration of loop() and whether they do or not (and the direction) is determined by the values in some global variables that have been updated by the switches.
If it was my project all I would have in loop() would be
void loop() {
readSwitches();
moveMotors();
}
And please go back to edit your original post and correct the spelling mistake in the title. It should be "simultaneous"
...R