How to catch key operation signals send from the master host to the sub host in dual camera setup?

[LogWell]

My goal: I want to take a few images with two cameras without recording the whole video.

My setup:

PC 1 ~ Device 1 (set as master)
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|    a 3.5-mm audio cable
|
PC 2 ~ Device 2 (set as sub)

The difference from the document is that I use code (C++) to control two devices (first turn on machine 2 and then run machine 1).
I can capture images through custom keys on PC 1, but how can I pass this keyboard operation through the cable to PC 2 so that I can capture images simultaneously?