Yeah maybe they say that, but I think it's only with the natives apps, included with Windows 8. They are many apps that can manage the two camera at the same time in the windows Store, and also many website through flash.
For example, In the Windows Store, you can dowload and install CyberLink "You cam mobile" and the app has the option to use the 2 camera simultanously and it's literally named "Double Camera"
. There are others too.
Also, I would like to have at least one of the two camera working with icamsource. Anyway, the app Icamsource does'nt Use the two camera at the same time ! You chose one or another in the interface. I don't want to use the two at the same time, at least use one of them by swithching according to the need