What version of the Arduino IDE are you using? If you need to communicate with two Edison’s using Tx and Rx you must use Serial and Serial1. I can see in your code you are only using Serial. The Serial object is used for the Serial monitor. So you can use this in your receiver board sketch. Serial1 is used to communicate with serial devices; you can communicate and send data with Serial1.write function. You’d have to edit the serial object in your second code, the sender, to use Serial1 instead of Serial.