programa para hacer funcionar 2 sensores ultrasonicos srf04 con arduino

hice un programa en el cual incluyo dos sensores ultrasonicos, pero a la hora de correrlo solo me funciona uno, y los probe por separado y si funcionan los dos, espero y me puedan ayudar a corregir el programa, decirme en que esta mal,o decirme como se hace, o si me pudieran pasar ejemplos de otros programas que funcionen con dos o mas sensores se los agradeceria.
les adjunto el programa en archivo y en texto.
int led1=13;
int led2=8;
int pingPin = 7;
int pingPin2 = 6;
int value = 0; // Valor del pulsador
int value2 =0;

void setup() {
// initialize serial communication:
Serial.begin(9600);
}

void loop()
{
// establish variables for duration of the ping,
// and the distance result in inches and centimeters:
long duration,duration2, cm,cm2;

// The PING))) is triggered by a HIGH pulse of 2 or more microseconds.
// Give a short LOW pulse beforehand to ensure a clean HIGH pulse:
pinMode(pingPin, OUTPUT);
digitalWrite(pingPin, LOW);
delayMicroseconds(2);
digitalWrite(pingPin, HIGH);
delayMicroseconds(5);
digitalWrite(pingPin, LOW);
// The same pin is used to read the signal from the PING))): a HIGH
// pulse whose duration is the time (in microseconds) from the sending
// of the ping to the reception of its echo off of an object.
pinMode(pingPin, INPUT);
duration = pulseIn(pingPin, HIGH);

pinMode(pingPin2, OUTPUT);
digitalWrite(pingPin2, LOW);
delayMicroseconds(2);
digitalWrite(pingPin2, HIGH);
delayMicroseconds(5);
digitalWrite(pingPin2, LOW);
// The same pin is used to read the signal from the PING))): a HIGH
// pulse whose duration is the time (in microseconds) from the sending
// of the ping to the reception of its echo off of an object.
pinMode(pingPin2, INPUT);
duration2 = pulseIn(pingPin2, HIGH);

// convert the time into a distance

cm = microsecondsToCentimeters(duration);
cm2 = microsecondsToCentimeters(duration2);
value = duration / 29 / 2;
value2 = duration2 / 29 / 2;

pinMode(led1, OUTPUT);
pinMode(led2, OUTPUT);

Serial.print(cm);
Serial.print(“cm”);
Serial.println();
delay(100);

Serial.print(cm2);
Serial.print(“cm2”);
Serial.println();
delay(100);

if
( cm <=20 )

{digitalWrite (led1,HIGH);}
else
{digitalWrite (led2,LOW);}

if
( value2 <= 20)
{digitalWrite (led2,HIGH);
}
else
{digitalWrite (led2,LOW);}

}

long microsecondsToCentimeters(long microseconds)
{
// The speed of sound is 340 m/s or 29 microseconds per centimeter.
// The ping travels out and back, so to find the distance of the
// object we take half of the distance travelled.
return microseconds / 29 / 2;
}

_9_de_octubre_prueba_ultrasonidos.ino (2.18 KB)