If you really need fog lights, having both on is a good thing, IMHO. Problem is, many people just seem to run around with them on all of the time, especially those on the front (not available on the i3 unless you add some). I just look at the cars in front. When I have trouble seeing their taillights, I turn my fog lamps on, and as things clear up, turn them off. On a trip, that may mean doing it multiple times. All part of being aware of the conditions. In most cases, running around town in stop and go traffic, they aren't often that useful. If they are, and it blinds someone behind, they're following too closely, anyway!
I don't know if the control logic is there to only turn one of them on. In my GT, the logic was already in the vehicle...all I had to do was swap the switch. The message will still go out on the i3, but the central processor, or whichever one that decodes it, isn't programmed out of the factory to respond. Since I tend not to drive my i3 when the weather is bad enough to make rear fog lights useful, I didn't buy a new switch for it to enable the function.
2011 535i x-drive GT, 2014 i3 BEV