Two Ultrasonic Sensor!?

I modify the program to follow:

#include <Servo.h> 

Servo myservo; 


int pos1 = 90; 
int pos2 = 150;
int pos3 = 30;
int number = 0;
int decrease =0;



void setup() {
myservo.attach(10); 
pinMode(7,   OUTPUT);
pinMode(11,   OUTPUT);
pinMode(13, OUTPUT);
pinMode(12, INPUT);
}

void loop()
{
  long front; 

//  pinMode(pingPin, OUTPUT);
  digitalWrite(13, LOW);
  delayMicroseconds(2);
  digitalWrite(13, HIGH);
  delayMicroseconds(10);
  digitalWrite(13, LOW);

//  pinMode(inPin, INPUT);
  front = pulseIn(12, HIGH);
  
  delay(100);



if (front <= 2900)
{  
    analogWrite(11, 0);
    delay(50);
    digitalWrite(7, HIGH);     
    delay(50);
    analogWrite(11, 90);
    delay(600);
    

        myservo.write(pos2);
    delay(100);
    delay(600);

    myservo.write(pos1);
    delay(100);
    number = 1;
    decrease = 0;
}

else if (front > 2900 && front <= 11600)
{
if (number = 1)
{  digitalWrite(7, LOW); 
    analogWrite(11,95);
      delay(30);
}    
if (decrease >= 50)
{
    digitalWrite(7, LOW); 
    analogWrite(11,45);
      delay(15);
          myservo.write(pos1);
}
else
{
  digitalWrite(7, LOW); 
    analogWrite(11,75);
      delay(15);
          myservo.write(pos1);
decrease = decrease + 1;
}
}

else 
{
  digitalWrite(7, LOW); 
    analogWrite(11, 130);
      delay(15);
          myservo.write(pos1);
decrease = decrease + 1;
}

}

First, There are two ultrasonic sensor, i call them front ultrasonic and back ultrasonic.
Pin 12,13 Front Ultrasonic
Pin 8,9 Back Ultrasonic
This program only use the front ultrasonic
But i don't know why there are some problems
1. When i connect both ultrasonic sensor without changing the program. The back ultrasonic work same as the front ultrasonic.
That means when back ultrasonic detect object, the car move backward.
2. When i connect only the back ultrasonic vcc and ground to same voltage source, same problem
3. When i connect the back ultrasonic vcc and ground to an external voltage source, same problem
Maybe you would ask me why use this program rather than two ultrasonic program
Because i wanna step by step, If only connect front ultrasonic, it works perfect,
But i really don't understand why if i connect the other ultrasonic sensor to it, it also works
I would be happy if anyone can help me to fix this problem