I had some breakthrough yesterday:
I managed to get both GPIO joysticks to be js0 and js1, while my xbox controller became js2! This was the requirement to use these joysticks with the hardcoded input mapping of UAE4ALL2.
What I did:
1. I installed the xbox-driver as daemon, using step 3 of >this tutorial<
2. Then I blacklisted the xpad driver (the default Linux USB driver for X-Box gamepads). Something I read in the manpage of xboxdrv:
Add the line “blacklist xpad” into file /etc/modprobe.d/raspi-blacklist.conf
3. After that I only had put the gpio driver on first place in /etc/modules
xboxdrv works much better than the kernel xpad driver, because it has much more options. It has the possibilty to define a deadzone, which made some problems with xpad inside UAE4ALL2, because js0 and mouse are used both simultanously on the first amiga port. Now I can click the right mouse button for some trainer options at gamestart, while I can use the first GPIO joystick inside of the game itself.
I had problems with UAE4ARM, but there was no time to dig deeper into that. There seems to be some additional bugs in the joystick code.
But in UAE4ALL2 (chips version 5) everything worked perfectly. The same in Vice.
There is still one expected sideffect: All retroarch emulators and controls need to be reconfigured. So there is still some work to do.
Nevertheless I hope chips will come up with a proper joystick gui and mapping someday. That would make things much easier and beginner friendly.