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From looking at your first image it seems that you have the ArduinoA with GRBL connected to your PC and your ArduinoB is connected to the GRBL ArduinoA. What on earth are you trying to do?
And your diagram does not show any of the Arduino pin numbers. If your ArduinoB is connected to the same Serial port on ArduinoA as the PC is using it is not surprising if things go wrong.
And if GRBL is getting commands from the PC I suspect it is going to be confused if it also gets commands from another source.
This is one the best examples I have seen of the XY Problem. You really need to explain what you want to achieve and not start with some detail question about how you think it might be done
...R