Why usb xhci port just recognize echi device with "Y‘ cable

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I made a cable like "Y", one is for echi port(usb2.0), another for xchi port(usb3.0).
I put this cable in a xchi port of a linux server(Redhat 7.4 x86-64), and one of the Y side is plugging USB2.0 stick, another is USB3.0 stick.

The I power up my linux server with AC, find only usb2.0 device found in linux with "fdisk -l". But I can find both USB2.0 and 3.0 stick if I remove USB3.0 device and replug in. So what is the reason ?

Thanks a lot.


Can I enable USB2.0 and USB3.0 for the same xHCI port in the same time after linux boot up ?