These Toshiba screens use over 5W (the maximum power that can be delivered by the USB standard). They are shipped with a dual cable so you can use the power from 2 USB ports to power the screen.
However most USB ports will allow more than 5W to be delivered, so you can probably get away with 1 USB connection for a screen and get enough power.
However if you run it through an unpowered hub, you will only have 5W (maybe a bit more) for all the hub ports. This will not be enough for 2 of these Toshiba displays. I'm surprised 1 works when you consider that the USB hub also needs power.
You will need a powered USB hub to make this work, and make sure that the USB hub can deliver 5W out of all hub ports when fully loaded.
Wim
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