Tx/Rx UART communication between 2 MCU over 1 line
While your assumption of how the RX works is correct, care needs to be taken with the two TX driving the same line. You want to at least buffer the TX with a reverse diode and a pull-up resistor, like this:



Doing something like this is nothing new and there are lots of references on the internet. This configuration will actually be useful to help detect transmission errors as the transmitter also receives the loop back data.

You might want to look up some stuff on the internet such as this and this.