From what I can tell, referencing the book Practical Computer Vision with Simple CV...
On Linux, all peripheral devices have a file created for them in the /dev directory. For cameras, the file names start with video and end with the camera ID, such as /dev/video0 and /dev/video1 . The number at the end equals the camera ID.
And as far as telling which camera number goes to which device (also from the book)...
from SimpleCV import Camera
# First attached camera
cam0 = Camera(0)
# Second attached camera
cam1 = Camera(1)
# Show a picture from the first camera
img0 = cam0.getImage()
img0.drawText("I am Camera ID 0")
img0.show()
# Show a picture from the first camera
img1 = cam1.getImage()
img1.drawText("I am Camera ID 1")
img1.show()