Agreed. If a two port USB hub worked at all we would likely be limited to a USB device in one place or the other but not both simo.
Alternatively, I'm thinking a USB extension cable with a 90 degree plug could come from the rear of the dash cubby hole next to the USB jack and just plug in (see thumbnail). It could be unplugged from time to time and coiled up in the cubby hole if something else needs to plug in that front jack. The other end goes into the floor super console top box as you suggested. This approach shouldn't have warranty issues.
Either way it needs to be "professionally dressed" and easily removed or tucked/hidden during trips to the shop.
much easier to add a cigarette lighter and connect it to a 12v source then plug in a usb cig lightr so you can have usb, this can be hidden under the seats