Connect Serial1 (RX1,TX1) of DUE "Master" to Serial1 (TX1, RX1) of DUE "Slave1", and Serial2 (RX2,TX2) of DUE "Master" to Serial1 (TX1, RX1) of DUE "Slave2".
You can find example sketches for using Serial.available() in File>Example>Communication.
An example sketch to play with Serial1, Serial2 on a single board. It's easy to cut the sketch for 2 DUE boards (don't forget to connect GND pins together !):
/*********************************************************************/
/* Jumper between RX1 and TX2, and another one between TX1 and RX2 */
/*********************************************************************/
char c = 0;
void setup() {
Serial.begin(250000);
Serial1.begin(5000000);
Serial2.begin(5000000);
Serial2.print("Hello");
}
void loop() {
String s;
s.reserve(10);
s = "";
while (Serial1.available() > 0) {
c = Serial1.read();
s += c;
}
if (s.length() > 0) {
Serial.println(s);
Serial2.print(s);
}
delay(1000);
}