i need to let 2 motors turn but not at the same time

Projects Programming
3 
 if (buttonState == HIGH)
   if (buttonState2 == LOW) {
    // motor Rechts:
    digitalWrite(8, HIGH);
    digitalWrite(9, LOW);
    digitalWrite(10, LOW);
    digitalWrite(11, LOW);
}
else {
    // Motor uit:
    digitalWrite(8, LOW);
    digitalWrite(9, LOW);
    digitalWrite(10, LOW);
    digitalWrite(11, LOW);
  }
  if (buttonState == HIGH)
  if (buttonState2 == LOW) {
    // motor Rechts:
    digitalWrite(8, LOW);
    digitalWrite(9, HIGH);
    digitalWrite(10, LOW);
    digitalWrite(11, LOW);
}
else {
    // Motor uit:
    digitalWrite(8, LOW);
    digitalWrite(9, LOW);
    digitalWrite(10, LOW);
    digitalWrite(11, LOW);
  }

Bearing in mind that digitalWrite takes just a few microseconds per call, you are not going to have time to notice the effect of the first set of conditions, before they are overwritten by the second.