This is the first time I EVER had a fog light of any kind on my car, so I was eager to try it out in my garage. Only ONE lamp, on the (American) driver's side.
The Brits are more familiar with fog lights on the REAR of their cars than we Americans. On their Jaguar forum, they were wondering the same thing---why only one fog-lamp.
The theory in Europe is that, TWO lamps seen by a trailing driver might be mistaken for "break lights", and confuse him in the fog. That driver needs to see your fog light as just a marker, and still immediately understand your break-lights when THEY come on.
... I'm not sure I want to agree with that theory, because it would have been cool to have two fog lamps on the back of my Jag. :|I feel like a bicycle rider with just one.