You are unlucky with 2 player adapters. This time Linux does not support the Arduino with 2 controllers. It would be easier to just use 2 adapters
But then you have to make sure the serial is different for both.
Unlucky, yes you can say that
I do have a other pro micro laying around and yesterday i was at the point of doing what you suggested. But I wanted to wait out any reply here befor doing that. In the end i hate to use more usb ports because of something bit silly. But then again if it works it works. Bigger usb hub in not the end of the world.
About hid serial with a different board you can easly change it with in the boards.txt
Again i am thank full for everything you tryed.