Alternate between 2 relays with one button
The way you have it, both if blocks will be evaluated each time you call to loop(). Instead, you need to create a variable that remains "alive" in between each call to loop() to keep track of which relay you want to activate:

const int ledPin = 2;
const int ledPinA = 3;
int active_relay = 2;   // State variable for current relay pin

void setup() {
    pinMode(ledPin, OUTPUT);
    pinMode(ledPinA, OUTPUT);
    Serial.begin(9600);
}

void loop() {
      int sensorValue = analogRead(A0);

      Serial.println(sensorValue);

      delay(1);    // This delays for 1ms, not 1s ??

      if (sensorValue &gt; 500) {   // Pulse received
          digitalWrite(active_relay, HIGH);  // Relay on
          delay(1000);                       // Wait 1 second
          digitalWrite(active_relay, LOW);   // Relay off

          // Update variable for next call to loop
          if (active_relay == ledPin) {
              active_relay = ledPinA;      // Swap
          }
          else {
              active_relay = ledPin;       // Swap
          }
      }
      // Remove 2nd if block
}