// Define client addresses
#define CLIENT1 0x01
#define CLIENT2 0x02

// Define client commands
#define STOP  0x04
#define START 0x08

void setup() {
  Serial.begin(9600);
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  // Turn LED off (to indicate client1)
  digitalWrite(LED_BUILTIN, HIGH);  
  // Send command START to CLIENT1
  Serial.print(CLIENT1, BIN); // Send address for client 1
  Serial.print(START, BIN);   // Send the command
  // Wait 5 seconds
  delay(5000);
  // Turn LED on (to indicate client2)
  digitalWrite(LED_BUILTIN, LOW);  
  // Send command START to CLIENT2
  Serial.print(CLIENT2, BIN); // Send address for client 2
  Serial.print(START, BIN);   // Send the command
  // Wait 5 seconds
  delay(5000);
}