volatile bool blinkMode = false; 

void setup() {
  pinMode(2, INPUT);   
  pinMode(13, OUTPUT); 
  pinMode(12, OUTPUT); 
  
  attachInterrupt(digitalPinToInterrupt(2), toggleMode, CHANGE);  
}

void toggleMode() {
  blinkMode = !blinkMode;  
}

void gleichblinker() {
  digitalWrite(12, HIGH);  
  digitalWrite(13, HIGH);  
  delay(500);
  digitalWrite(12, LOW);   
  digitalWrite(13, LOW);   
  delay(500);
}

void wechselblinker() {
  digitalWrite(12, HIGH);  
  digitalWrite(13, LOW);   
  delay(500);
  digitalWrite(12, LOW);   
  digitalWrite(13, HIGH);  
  delay(500);
}

void loop() {
  if (blinkMode) {
    gleichblinker(); 
  } else {
    wechselblinker(); 
  }
}