Porsche 911s in the late 60s and early 70s had
a dual battery setup.  I had a '72 911 for 20
years.

Everything is fine until one battery dies--one
cell goes bad.  Then it kills the other.  Rather,
the other keeps trying to save the bad one.
Anyway, by replacing both batteries together,
it all works fine.  Porsche did it to balance the
weight.  The batteries were "petite" and they
wanted two for cranking power.

BTW--my '88 Carrera has one GIANT battery.  They
built up the weight of the car to the point that
batteries didn't matter.  ;-)

                        Richard A Jones
                        Boulder, Colorado