Connecting 2 arduino boards together.

Projects General Guidance
1

Hi, I'm trying to connect 2 Due's using the tutorial "Arduino to Arduino Serial Connection" -> Arduino to Arduino Serial Communication | Robotic Controls. I get the Physical pixel tutorial to play on one board connected to one computer but when I connect the other Due to a different computer and put this code in :

void setup() {
  Serial.begin(9600);
}

void loop() {
  Serial.print('H');
  delay(1000);
  Serial.print('L');
  delay(1000);

It says "No device on COM4" This only happens when I connect both the TX and the RX from one board to another. What can I do to fix this.
Thanx

2

Do not connect them together during upload of code because the other arduino is swampling the signal from the USB and so the computer things there is nothing connected.