#include <Wire.h>
#include <SoftwareSerial.h>                  

#define SLAVE_1 1
#define SLAVE_2 5
   
SoftwareSerial mySerial(3, 4); // RX, TX    

void setup() {
  Wire.begin(); // join i2c bus (address optional for master)
  mySerial.begin(9600);
  mySerial.println("start");
}

byte x = 0;

void loop() {
  Wire.beginTransmission(SLAVE_1); // transmit to address
  Wire.write(x);              // sends one byte
  Wire.endTransmission();    // stop transmitting
  delay(1000);
  Wire.beginTransmission(SLAVE_2); // transmit to address
  Wire.write(x);              // sends one byte
  Wire.endTransmission();    // stop transmitting
  delay(1000);
  x = (x == 0) ? 1 : 0;
}