@nilesh just add some delay between two massege

I modified your code check it

const int Main1 = 8;
const int Main2 = 9;
const int Store1 = 7;
const int Store2 = 10;
bool i = false;
bool j = false;
bool k = false;
bool l = false;

void setup()
{
  Serial.begin(9600);

  pinMode(Main1, INPUT);
  pinMode(Main2, INPUT);
  pinMode(Store1, INPUT);
  pinMode(Store2, INPUT);

  digitalWrite(Main1, LOW);
  digitalWrite(Main2, LOW);
  digitalWrite(Store1, LOW);
  digitalWrite(Store2, LOW);

  delay(1000);
}
void loop()

{
  sensor1();
  sensor2();
}
void sensor1()
{
  if (digitalRead(Main1) == HIGH && digitalRead(Main2) == LOW && i == false)

  {
    Serial.println("OK");
    delay(1000);
    Serial.print("AT+CMGF=1\r\n");
    delay(500);
    Serial.print("AT+CMGS=\"");
    Serial.print("+91-----------");// first number
    Serial.print("\"\r\n");
    delay(2000);
    Serial.print("Main gate closed");
    Serial.write(0x1A);
    delay(5000);
    Serial.print("AT+CMGS=\"");
    Serial.print("+91-----------");// Second number
    Serial.print("\"\r\n");
    delay(2000);
    Serial.print("Main gate closed");
    Serial.write(0x1A);
    delay(1000);
    i = true;
    j = false;

  }
  if (digitalRead(Main2) == HIGH && digitalRead(Main1) == LOW && j == false)
  {
    Serial.println("OK");
    delay(1000);
    Serial.print("AT+CMGF=1\r\n");
    delay(500);
    Serial.print("AT+CMGS=\"");
    Serial.print("+91---------------");//first number
    Serial.print("\"\r\n");
    delay(2000);
    Serial.print("Main gate Open");
    Serial.write(0x1A);
    delay(5000);
    Serial.print("AT+CMGS=\"");
    Serial.print("+91--------------"); second number
    Serial.print("\"\r\n");
    delay(2000);
    Serial.print("Main gate Open");
    Serial.write(0x1A);
    delay(1000);
    j = true;
    i = false;
  }
}
void sensor2()
{
  if (digitalRead(Store1) == HIGH && digitalRead(Store2) == LOW && k == false)
  {
    Serial.println("OK");
    delay(1000);
    Serial.print("AT+CMGF=1\r\n");
    delay(500);
    Serial.print("AT+CMGS=\"");
   Serial.print("+91-----------");// first number
    Serial.print("\"\r\n");
    delay(2000);
    Serial.print("Store room is closed");
    Serial.write(0x1A);
    delay(10000);
    Serial.print("AT+CMGS=\"");
    Serial.print("+91-----------");// second number
    Serial.print("\"\r\n");
    delay(2000);
    Serial.print("Store room is Closed");
    Serial.write(0x1A);
    delay(1000);
    k = true;
    l = false;
  }
  if (digitalRead(Store2) == HIGH && digitalRead(Store1) == LOW && l == false)
  {
    Serial.println("OK");
    delay(1000);
    Serial.print("AT+CMGF=1\r\n");
    delay(500);
    Serial.print("AT+CMGS=\"");
   Serial.print("+91-----------");// first number
    Serial.print("\"\r\n");
    delay(2000);
    Serial.print("Store room is Open");
    Serial.write(0x1A);
    delay(5000);
    Serial.print("AT+CMGS=\"");
   Serial.print("+91-----------");// Second number
    Serial.print("\"\r\n");
    delay(2000);
    Serial.print("Store room is Open");
    Serial.write(0x1A);
    delay(1000);
    l = true;
    k = false;
  }
}

thanks