Did you create sys-usb yourself or it was set up automatically during the installation? Did you read this? Do you have a USB keyboard? Do you have your usb controllers attached to sys-usb (see its Settings/Devices)?
You should have the dom0 icon (widget) even if sys-usb functions correctly. What does command qvm-usb show in dom0?
It probably means that one your USB controller is (correctly) attached to the sys-usb, whereas the other isn’t. Have a look at the sys-usb Preferences/Devices and find the second controller in the left list and put it to the right. By the way, if you really have two separate USB controllers, it might be a good idea to have separate sys-usb qubes for them, for more isolation.