Two theories on why the 8 has two batteries.
First is the weight and balance theory that says the two batteries are the finishing touch on nearly perfect weight distribution front to back and side to side.
The second is that the two huge batteries and the 140 Amp alternator are there just to power all of the electronic toys that the engineers could stuff into the the car. After all it has "computers" in the door sills because they ran out of space in the conventional places.
When you close the door and lock the car it still takes about 15 minutes before everything is shut down. E32 7 series is almost as trippy as the E31 8 series because they shared engineering to a large degree.