When you have two Snaps installed side-by-side and facing the same direction, and if there are objects in front of both, the Infrared light from one Snap can be reflected into the lens of the second Snap by the nearby objects.
This causes the second Snap to not go into night vision mode, while its infrared light causes live video to look purple at the same time. Pointing Snaps at different directions can often solve the problems.