Easy way is probably two usb-serial dongles and a null modem, if you don't need huge amounts of throughput.
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(I would use it to use a laptop keyboard on a headless computers).
I am looking for an easy solution, it does not have to be the cheapest.
ChatGPT points me to Arduino, but as far as I can see, there's no arduino with 2 usb ports. It also points me to Raspery pi zero, but that's a computer, not a microcontroller, so not sure if it's suitable.
If anyone with experience can give me some pointers, it would be greatly appreciated!