Stepper motor is only moving one step at a time with an IR sensor
As chrisl said, you need to call run repeatedly.

I tried to rewrite the code you've given, though I'm not entirely sure what it is you try to achieve.

void loop() 
{
  stepper.run();  // Actually makes stepper move
  stepper2.run();  


  int curr_state = digitalRead(pin_ir_in);
  if (curr_state != prev_state)
  {
    if (curr_state == 0) // Motion sensor activated
    {
      stepper.moveTo(100);
      stepper.setMaxSpeed(200);
      stepper.setAcceleration(100);
      stepper2.moveTo(100);
      stepper2.setMaxSpeed(200);
      stepper2.setAcceleration(100);
    }
    prev_state = curr_state;
  }
}