https://github.com/OpenKinect/libfreenect2/issues/715
It can be done with a usb 3.0 pci express cards but you will need a powerful machine to do so.
You will need different USB controllers for each of the kinects. At least that is what I have found from my research.
I am also hoping for something like this https://youtu.be/1bodwMQ2zv8.
Where you use another computer to send data to the main machine.