Hello @djsmo
you have to check a good romset with the right core.
Check the arcade documentation.
https://wiki.recalbox.com/en/home

Thanks for your answer @dh04000 !
But Recalbox seems to recognize it as a "Twin USB Joystick". If I plug the two blue adapters I have, RecalBox recognizes 4 controllers... If I map the buttons on the controller plugged in the 2nd USB port, Recalbox configures the 3rd controller. So that all seems logical. The issue seems somewhere else.