Thank you Chris,
you know that (tr)uSDX (not trSDX) software/firmware is not open source?
Mmmm... yes, why? Did I say something similar?
(thx for correcting me with typo)
So what do you want to do with an extra serial port on an 8-Bit Microcontroller doing a hard job in software defined radio? It already has one as you stated....

Thank you Chris to ask the question: my intention is to study solution to remote control (tr)uSDX from Arduino board (or such kind of board - ESPx STM32 familly... whatever). The (tr)uSDX serial is exposed via USB which requires extra drivers on the other side (as far as I understand). I am not aware such kind of board could be able to read/write serial port between their USB ports, again I might be wrong.
This is why in first intention I was looking for serial connection between pin header of boards from each side.
This is why I ask here, in an open mind way
73,
Léopold