In some cases you maybe need combine sys-usb with sys-net. But it's less secure than having sys-net and sys-usb separate, so be aware of it before doing it. To do it :
nano /etc/qubes-rpc/policy/qubes.InputKeyboard nano /etc/qubes-rpc/policy/qubes.InputMouse
replace from
sys-usb dom0 allow,user=root
to
sys-net dom0 allow,user=root
Go in the sys-net terminal and shut it down, for exemple with init 0 Go in the sys-usb terminal and shut it down, for exemple with init 0
Go in the settings of your sys-usb Qube and disable "start on boot" and apply your modification Go in the settings of your sys-net Qube, and in the devices section, find the usb controler of your computer, selected it and apply your modification.