How to get values from 2 ultrasonic sensors

if i put in the code below, i only get the value for 1 of the 2 ultrasonic sensor. The other sensor gives a 0 value continuously.
I need both of the measured distance value. what should i do?

const int ping1Pin = 9; // Trigger Pin of Ultrasonic Sensor
const int echo1Pin = 10; // Echo Pin of Ultrasonic Sensor
const int ping2Pin = 11; // Trigger Pin of Ultrasonic Sensor
const int echo2Pin = 12; // Echo Pin of Ultrasonic Sensor

void setup() {
// put your setup code here, to run once:

// ultrasoon
pinMode(ping1Pin, OUTPUT);
pinMode(echo1Pin, INPUT);
pinMode(ping2Pin, OUTPUT);
pinMode(echo2Pin, INPUT);
}

void loop() {
// put your main code here, to run repeatedly:

// uitlezen ultrasoon
long duration1, x1;
digitalWrite(ping1Pin, LOW);
delayMicroseconds(2);
digitalWrite(ping1Pin, HIGH);
delayMicroseconds(10);
digitalWrite(ping1Pin, LOW);
duration1 = pulseIn(echo1Pin, HIGH);
x1 = microsecondsToMillimeters(duration1);
delay(10);

long duration2, x2;
digitalWrite(ping2Pin, LOW);
delayMicroseconds(2);
digitalWrite(ping2Pin, HIGH);
delayMicroseconds(10);
digitalWrite(ping2Pin, LOW);
duration2 = pulseIn(echo2Pin, HIGH);
x2 = microsecondsToMillimeters(duration2);
delay(10);

} // EINDE HOOFDPROGRAMMA

long microsecondsToMillimeters(long microseconds) {
return microseconds / 2.9 / 2;
}

How are you reading the output of the sketch? I see no way to know what the program is outputting. No serial prints or LCD or ...

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