let me share my Super Pro Mini Stereo Serial Arduino communicator expert
That device plugs into TWO usb ports and provide TWO simultaneous communication ports.
It consists of one Arduino Pro Mini and two USB-UART converters + a bit of soldering (not much):
Basically you bend the 5 programming pins upright (It had, instead of the two GND pins, a pin 3,3v that I had to cut away).
Put and solder the Arduino pro mini on the converter pins (take care of having DTR on the right side)
Cut away the exceding length fr the pins DTR, TX, RX leave only +5V and GND at full length.
Put and solder the second USB-UART converter and solder +5V and GND (for mechanical stabilty)
Solder a wire between TX from the second USB-UART converter and e.g. pin4
Solder a wire between RX from the second USB-UART converter and e.g. pin5
Ready! It took me 5 minutes to do it.
With it you can do real nice things:
You can have a communication channel open for your main purpose and have the serial monitor/plotter to debug/control/visualize your pocess/recompile.
Here an example at work:
The second USB/UART converter is connected to 2 IO pins and driven by software serial.
It does NOT reset the Arduino when you start/switch the serial monitor/plotter.
n.b.: You should purchase USB-UART converters having directy an USB connector on it and do it my way:
with soft cables, you might plug the ports differently and get a huge mess with the port names.