unsigned long prevMillis = 0;

void setup() {
  Serial.begin(9600);
  prevMillis = millis();
}

int state = 0;
void A() {
  static int count = 10;
  if(millis() - prevMillis >= 1000) {
    prevMillis = millis();
    count--;
    Serial.println(count--);
  }
  if (count <= 0) {
    count = 10;
    state = 1;
  }
}
void B() {
  static int count = 5;
  count--;
  if(millis() - prevMillis >= 1000) {
    prevMillis = millis();
    count--;
    Serial.println(count--);
  }
  if (count <= 0) {
    count = 5;
    state = 0;
  }
}

void loop () {
  switch (state) {
    case 0:
      A();
      break;
    case 1:
      B();
      break;
  }
}