Two webcams on the same hub won't work.
I believe Woland meant to say two identical webcams on the same USB hub won't work.
This is a matter of the hardware having machining IDs, so it creates a conflict. Moving one to another USB hub causes each to have a unique address.
NOTE: not the next slot, but to another collection of slots. EG, you have 2 USB slots on the left of your laptop, and 3 on the right.. this is likely 2 hubs, so you will need one connected to each side