do you have both boards plugged in?
if so you may need to use 'select device' to select the other board.
btw, I got my boards today, had some teething troubles using all the boards... I think partly user error (e.g. getting confused which boards Id plugged in etc )
but what I did to get going, and troubleshoot, was after testing the first board was working, I deleted the midi device (using audio midi setup) , and plugged in the new board, and restarted axoloti GUI. to test that board..
I don't think it was necessary but just helped me verify each board individually.
once that was working, I plugged in each board and then renamed them to Axo 1, Axo 2 etc, so I can see them independently, and select them as needed.