We would need to see a logfile from a recording attempt, to give more accurate information.
A USB hub wouldn't inherently cause an issue, much more likely would be that the two cameras are using too much USB bus bandwidth.
Alternately, if you bought two identical cameras, many cheaper options (and Logitech) don't deal well with having more than one of the same camera plugged into the same system. It's especially bad on the cheaper no-name brands, whose firmware can have stuff like ID enumeration hardcoded, rather than dynamically allocated (an 'I am Spartacus!' problem).
Thirdly, if they're identical, try both in the VCD Source config dropdown. Most video capture devices can only be used by one program at a time, and they don't necessarily show up in the same order every time in the list with identical-names. And each VCD Source counts as a separate 'program'. So if you're picking the same camera twice, one will show a blackscreen. Kind of like how if you use 'add new' on a second scene instead of 'add existing' to add a camera in-use in the first scene, you'll run into problems where the camera 'disappears'.
Short version, need more information to give more accurate troubleshooting. :)
