Since both batteries are tied together at the (+) solenoid stud, both
receive equal charging?   The big red wire from the 90 amp alternator goes
directly to this stud.  Both batteries are matched in size and age, which
helps.  I would not suggest using two different group batteries and I would
not mix lead/acid and deep-cycle.  I thought I would try it this way (with
the group 41s) because  my '89 Jetta diesel uses the same battery, so I
could always use it in there.  I do get better cranking speed.  I think the
wire size from the from the stock battery (at least in my '85) is a little
wimpy.

                                                        Jay