On the first RPi :
In a first terminal : (create FIFO)
In a second terminal : (Write datas into FIFO)
readfifo-writeUART.py : pyserial short-intro
with serial.Serial() as ser:
ser.baudrate = 19200
One of the most error prone issues is to use deprecated etc/rc.local. Please take note that using /etc/rc.local has limitations due to Compatibility with SysV. Following the recommendation of the developers from systemd you should avoid using it. We have seen many problems on this site if using it. An important note is the last sentence in the linked ...
If unconfigured GPIO pins are Inputs, so safe for a logic input 0 - 3.3V
If Output and opposite voltage is applied they can be damaged - which is why you should use a series resistor to limit current to a safe level.