volatile unsigned long time1 ;
volatile unsigned long time2 ;

volatile boolean valid1 = false ;
volatile boolean valid2 = false ;

void int1() {
  if (!valid1) {
    time1 = micros();
    valid1 = true ;
  }
}

void int2() {
  if (valid1 && !valid2) {
    time2 = micros();
    valid2 = true ;
  }
}

void setup() {
  attachInterrupt (0, int1, FALLING) ;
  attachInterrupt (1, int2, FALLING) ;
  Serial.begin(115200);
}

void loop() {
  if (valid2) {
    Serial.println (time2 - time1) ;
    noInterrupts () ;
    valid1 = valid2 = false ;
    interrupts () ;
  }
}