In my project I need to connect two teensy 3.6 boards via serial connection (currently using serial2) to transfer data. I have connected GND and the TX, RX pins and data transfer is working fine even at high speeds. However, only the board will start automatically that is connected first to USB (directly to PC or via a hub). The board that is connected later on will not start until I press the button on it (I toggle the onboard LED as the first instruction in the setup() function to confirm this). It works fine if I either remove the RX pin cable on the second board or remove the Serial2.begin() from the program on the first board. I have also tried using serial1 or serial3 but the issue remains.

Any idea on what I am doing wrong?