I have two sets of USB 3.0 magnetic drives, each set connected to its own USB 3.0 hub, each of which in turn is connected to its own USB 3.0 root hub on my host (where the bunch of devices are soft RAID:ed together).
The bandwidth of each "path" is saturated, in the sense that RAID data rate is limited by the root hub data rate, not by individual drives.
Let's say that I bought a 2-port USB 3.1 Gen-2 expansion card.
As far as I have understood, the two protocols use the same set of pins/wires (Reference: https://www.synopsys.com/designware-ip/technical-bulletin/achieving-10-gbps.html), where
USB 3.1 Gen 1 (USB 3.0) specifies a theoretical physical data rate of 5x1 Gbps.
USB 3.1 Gen 2 specifies a theoretical physical data rate of 5x2 Gbps.
Ignoring overhead such as discussed in the article, could I theoretically connect the two existing Gen-1 hubs to the single Gen-2 root hub, and fully utilize that card's bandwidth?
[EDIT] Ali Chen mentioned the use of "store-and-forward" in USB 3.1 G2 hubs, which led me to find the following:
"With Multiple INs, the USB 3.1 [Gen 2] host sends an IN request to both devices and the hub buffers the data coming from the 5Gbit/s USB 3.0 device and transmits it whenever there's capacity on the upstream port, maximising the use of the available bandwidth" http://www.techdesignforums.com/practice/technique/understanding-usb-3-1-protocol/