Hi MiDi,
thank you very much for your fast help. Yes, the guide isn't up to date and you cannot just copy and paste the commands, but it leads you very quick to the desired results.
It is similiar with your tip: I don't have this hotplug folder, but I edited the 98-gpib-generic.rules (there was already an example).
SUBSYSTEM=="usb", ACTION=="add|change", ATTRS{idVendor}=="0957", ATTRS{idProduct}=="0718", ATTRS{serial}=="MY53xxxxx1", ENV{GPIB_CONFIG_OPTIONS}="--minor 0"
SUBSYSTEM=="usb", ACTION=="add|change", ATTRS{idVendor}=="0957", ATTRS{idProduct}=="0718", ATTRS{serial}=="MY53xxxxx2", ENV{GPIB_CONFIG_OPTIONS}="--minor 1"
That helped with the adaptors itself and both are green now and dmesg is telling me that it attaches one with minor 0 and the other with minor 1, but the rest isn't working anymore. Python3 doesn't connect to any device and even ibtest isn't working anymore.
Sounds like a stupid mistake at some point...