Vi lascio il link ad un post dove viene spiegato il perché alcune volte non riusciamo ad acquisire da due webcam dello stesso modello.
http://www.dibiasi.it/exchweb/bin/redir.asp?URL=http://www.gamedev.net/topic/595137-capturing-video-from-multiple-usb-webcams-on-windows/
Riporto per velocizzare solo la parte principale...
Which version of the technology are you using?
VfW? WDM? EVR? DXVA? or DXVA2?
The really old VfW system only supported a single video stream. It has been dead for over a decade, but is supported by DirectShow. If your DirectShow interfaces show up as VfW then you need to switch drivers. Many cheap webcams still only support VfW drivers. If you have one of these ancient cameras, replace it.
Because VfW was a horrible standard, Microsoft quickly replaced with WDM. The hardware manufacturers didn't bother with the update. Many just wrote a WDM wrapper that called VfW under the hood. Most of these "Dual Mode" cameras retained the bug of only allowing a single input, but the WDM-only (single mode) cameras would support multiple cameras. If you have one of these cheap old "Dual Mode" cameras, replace it.
The DirectX version of DirectShow that relied on WDM was dropped back in '04. The WDM drivers have also been deprecated. They were present in original XP (from 9 years ago) so this might be the version you picked.
Since late 2007 a newer version has been part of the Windows SDK. The current cameras should have EVR or DXVA drivers. These should allow any number of cameras.