void setup() {
  // sets digital pin 7 as input to connect a switch
  pinMode(7, INPUT);  
  // sets the max number of tones to play to 2
  setTonePolyphony(2);
  // generates a 400Hz tone in output pin 8 with infinite duration
  tone(8, 400); 
}

void loop() {
  // if the switch is pressed 
  if (digitalRead(7) == HIGH) {  
    // and if 2 tones can be played simultaneously
    if (getTonePolyphony() >=2) {  
      // generate a 1000Hz tone in output pin 9 with infinite duration
      tone(9, 1000);
	}
  }
}