Porsche 911s from (about) 1967 to 1973 had a pair of batteries
in the front hooked in parallel.  I had a '72 for 20 years.
Porsche did this to add weights to  the front corners of the
car, so they could omit the lead weights they hid in the
bumpers to fix handling....

They work fine...until one starts to fail.  It then beats up
on the other and they both fail.  That's OK, since you should
replace them in pairs, as someone mentioned.Also, they should
be the same model, as was mentioned, too.  Oh, and since
they were "special" they cost a lot more....

$85 each from www.automotion.com.  :-)

                                Richard A. Jones
                                '81 Vanagon   Mr Bus
                                '87 Syncro Westy   El Jefe
                                '88 Carrera