What is the reason to have two alternators on the 400i?
As far as I know, the earliest cars to appear with two alternators were the US version of the 365 GT 2+2. The US cars apparently ran a bit hotter due to emission controls and they needed larger cooling fans - MUCH larger. The single (euro version) alternator did not provide enough power to run the fans and all the other accessories.
Most likely a space problem in that a single alternator of the right size wouldn't fit in the limited space available up front so the solution was to install two smaller ones on top of each other. Not an ideal solution for sure but, looking on the bright side, at least you can get home if one alternator fails!
The other thought is that the belt is too small to turn a bigger output alternator with out slipping.
But you get home! I didn't say with everything on. And, you can do it at night, I've done it with a broken belt.
