Hi Graboid, sorry for such a delay in getting back! First chance I have had to have a proper look at this. I have followed your steps and I am 100% wired up according to your wiring diagram. I add the usb quirks line and reboot. On rebooting, none of the buttons work at all. I have tried pushing select and x together (as well as every other combination of two buttons) and cannot access the retroarch gui. I tried hitting f1 on the keyboard too and this won't do anything either so I can't see any way of accessing the retroarch gui to even see whether it now sees it as two controllers. However Lstest does show it as two controllers...

any ideas? really appreciate your help and hopefully I can get this running! I am using a pi 3 and retropie v 4.1 (have tried older versions too). What are you using and what version of retropie are you using?