While the USB hub wont convert a USB 2 drive into a USB 3 one, theres something called bus contention. When two devices share a bus, in the absence of a bandwidth limiting method, and being both drives capable of using the full speed of the bus, both will divide the bandwidth by the ideal fraction. This means that the bus running at 480mbit/sec, both drives will see circa 240mbit/sec, minus overheads.
What most people dont understand - and this can be seen in the answers given - is that while the USB 3 hub cant make the USB 2 drives talk in USB 3 speed, it can surely eliminate bus contention by providing each drive with a separate USB 2 bus. That way each drive sees 480mbit/sec.
But, this depends on the way the hub was made. Same result can be obtained if you connet both drives in separate root hubs, ie, if you connect the drives to ports that are served by different chipsets or diferent ports within the same chipset that is able to handle each port independently, as if each port was a separate bus. If your motherboard provides, for example, four USB ports, it might happen that those four ports are shared by the same chipset, effectively making it similar to a single port plus a hub. But, if the motherboard happens to provide each port with a different root, as if each port was served by a single chipset (even if this chipset is physically present into a single integrated circuit that composes the motherboard) each port is then a separated and independent entity, thus being able to provide each 480mbit/sec independent of each other. This holds true for USB 3 hubs too. Their speed capability while operating those two USB drives depends on how they are internally constructed.
The solution using 3x USB 3.0 hubs is optimal, as the first hub collects data from two other hubs at max USB 3.0 speed, while each second hub in the chain provides the conversion from USB 2.0 to USB 3.0, but is not a cheap solution. Would be more economic wise to simply use two USB 3.0 hard disks, as even if the bandwidth is shared by two, USB 3.0 is so much faster than USB 2.0 that this shared approach is still faster than USB 2.0 drives running in independent and not shared USB 2.0 buses.