RaspberryPI / Motion & Multiple USB Cameras

Playing around with Motion on Raspian.  Motion was simple to setup with a single camera but when I added a second and switched to the thread config files I started to get :

Unable to query input 1 VIDIOC_ENUMINPUT: Invalid argument

Tracked it to a setting in the default thread2.conf

# The video input to be used (default: 8)
# Should normally be set to 1 for video/TV cards, and 8 for USB cameras
input 1

Change the input to 8 and all is well