The farbwerk is a USB 2.0 device but of course it can be also used with USB 3.0 (the linked adapter seems to be OK).
You can not connect two controllers to one contact. That is not possible in general for any USB device. You will have to use a USB hub
like this one for example.
There is no problem to connect the device to an external USB connector by using an adapter.
I am not sure about your fourth question. As said the farbwerk is a USB 2.0 device and will also run on a USB 3.0 interface of course.