Ok, so
this thread shows how to use 2 USB cameras connected to the roboRIO. Just remember to change the camera name strings accordingly.
To find the names of the cameras connected to the roboRIO, connect them both, turn on the roboRIO and access the web interface from a computer with a compatible browser (IE9, Mozilla Firefox). In the sidebar you'll see all the connected devices. Find the cameras and copy their names to the code. Look at the attached image in case of doubt.
Finally, in your SmartDashboard, insert a USB Webcam Viewer and you're done!