Commucinate two arduino with each other
Pins 0 and 1 on the Arduino Uno are used for serial communication with the computer over USB, including uploading sketches. If you connect anything to those pins you can interfere with the upload process.

To communicate between the two boards using serial communication without using pins 0 and 1 you can do software serial on other pins:
https://www.arduino.cc/en/Reference/SoftwareSerial