This will make SWD_CLK\SWD_DIO_PTA4 of Board A and B connect to KEA of Board B at the same time.
Please try to cut the line at the bottom of J8\J106\J015 for Board B.
And connect the GND signal of two Boards.

Please check the Name of OpenSDA on Board A, make sure it was select when debugging.
Best Regards,
Robin
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