How to send data to 2nd Arduino?

I got a main sketch going in my main arduino1 ; my question is how can I send that data to my second arduino2 so reads it? So arduino2 does the same thing on its outputs as arduino1? also is there a way where arduino2 sends it back to my main arduino1 for also it to replicate the data from arduino2?
please help me write a sketch!!! THANKS!! in Advance!

// Arduino pins used for the LEDs
#define LED1 11
#define LED2 10
#define LED3 9
#define LED4 8

// Arduino pins used for the switches
#define S1 5
#define S2 4
#define S3 3
#define S4 2

// State of each switch (0 or 1)
int s1state;
int s2state;
int s3state;
int s4state;

void setup() {
// pins for LEDs are outputs
pinMode(LED1, OUTPUT);
pinMode(LED2, OUTPUT);
pinMode(LED3, OUTPUT);
pinMode(LED4, OUTPUT);
// pins for switches are inputs
pinMode(S1, INPUT);
pinMode(S2, INPUT);
pinMode(S3, INPUT);
pinMode(S4, INPUT);
// setup serial port
Serial.begin(9600);
Serial.println("Serial port open");
}

void loop() {
s1state = digitalRead(S1);
digitalWrite(LED1, s1state);
s2state = digitalRead(S2);
digitalWrite(LED2, s2state);
s3state = digitalRead(S3);
digitalWrite(LED3, s3state);
s4state = digitalRead(S4);
digitalWrite(LED4, s4state);
Serial.print(s1state);
Serial.print(s2state);
Serial.print(s3state);
Serial.print(s4state);
Serial.println();
}

There are several ways to get two Arduinos to talk to each other - Serial, I2C, and SPI. Exactly how to do each one has been documented, if you'd bothered to search.

This sounds incredibly similar to this thread. See if you can get some ideas from that:

http://arduino.cc/forum/index.php/topic,102928.0.html

Oh, wait. YOU started that other thread. Right. I'm going to lock this one. Please stay with one thread.