int time1 = 0;
int time2 = 0;
int distance1 = 0;
int distance2 = 0;
int threshold = 10;
int counter = 0;
const int timeout = 3000;
bool flag1 = false;
bool flag2 = false;

void loop(){

  // get distances
  distance1 = getDistance(1);
  distance2 = getDistance(2);

  // check if distance from first sensor crosses threshold
  if(distance1 < threshold){
    // if so, record the time
    time1 = millis();
    // and set the triggered flag to true
    flag1 = true;
  }

  // check if distance from second sensor crosses threshold
  if(distance2 < threshold){
    // if so, record the time
    time2 = millis();
    // and set the triggered flag to true
    flag2 = true;
  }

// if BOTH triggered flags are true
  if(flag1 && flag2){
    // find the time between trigger events
    int difference = time1 - time2;
    // if the difference is positive, someone's going in, so increment counter
    if(difference > 0){
     counter++;
    }
    // otherwise subtract
    else{
     counter--;
    }
    // clear all the flags
  clearFlags();
  }
// if ONE sensor is triggered
  if(flag1 || flag2){
    // determine if one or the other has not been triggered for the timeout period
    if(millis()-time1 > timeout || millis()-time2 > timeout){
      // if longer than timeout, clear flags
      clearFlags();
    }
  }

}

float clearFlags(){
      time1 = 0;
      time2 = 0;
      flag1 = 0;
      flag2 = 0;
}

float getDistance (int whichSensor){

    int _trigPin;
    int _echoPin;

    if (whichSensor == 1){
    _trigPin = 4;
    _echoPin = 5;
    }

    if (whichSensor == 2){
      _trigPin = 2;
      _echoPin = 3;
      }

      digitalWrite(_trigPin, LOW);
      delay(2);
      digitalWrite(_trigPin, HIGH);
      delay(10);
      digitalWrite(_trigPin, LOW);

  int duration = pulseIn(_echoPin, HIGH);
  double distance = (duration/2) *0.000344;

  return distance;
}