const int ledPin9  = 9; // the number of the LED pin
const int ledPin10 = 10; // the number of the LED pin

const unsigned long interval9  = 1000; // interval at which to blink (milliseconds)
const unsigned long interval10 =  500; // interval at which to blink (milliseconds)

unsigned long LED09_Timer; // just a SINGLE variable for non-blocking timing
unsigned long LED10_Timer; 

void setup() {
  pinMode(ledPin9, OUTPUT); // set the digital pin as output
  pinMode(ledPin10, OUTPUT); // set the digital pin as output
  LED09_Timer = millis(); 
  LED10_Timer = millis(); 
}


void loop() {
  if ( TimePeriodIsOver(LED09_Timer,interval9) ) {
    digitalWrite(ledPin9,!digitalRead(ledPin9) ); 
  }

  if ( TimePeriodIsOver(LED10_Timer,interval10) ) {
    digitalWrite(ledPin10,!digitalRead(ledPin10) ); 
  }
}

// the attention mark "!" is the NOT-operator which inverts LOW to HIGH and HIGH to LOW
// digitalRead() can be done for outputs too  
// and it delivers the actual logic state 
// example digitalRead(ledPin9) = LOW    
// inverting with the NOT-operator !LOW results in HIGH
// which means digitalWrite(ledPin10,!digitalRead(ledPin10) )
// digitalWrite(ledPin10,!LOW )
// digitalWrite(ledPin10,HIGH )
// which toggles the state of the IO-pin

// easy to use helper-function for non-blocking timing
boolean TimePeriodIsOver (unsigned long &startOfPeriod, unsigned long TimePeriod) {
  unsigned long currentMillis  = millis();
  if ( currentMillis - startOfPeriod >= TimePeriod ) {
    // more time than TimePeriod has elapsed since last time if-condition was true
    startOfPeriod = currentMillis; // a new period starts right here so set new starttime
    return true;
  }
  else return false;            // actual TimePeriod is NOT yet over
}