Check with the installer of car audio systems.  They use multiple battery
installations on the big systems.  I got an apparatus that directs charge from
the alternator to whichever battery needs charge without overcharging either
one.  It is called an isolator.
 
  http://www.bcae1.com/battiso.htm  (this site gives good explanation of
various types)

Jim,

 

Thanks for the info.  My small (5AH) ignition backup battery is isolated by a diode (60 A low voltage drop Schottky).  One is not necessary for the 28 AH main battery since that is the main electrical system battery.  My batteries have different charging requirements but apparently they are close enough to that supplied by the single voltage regulator to not adversely affect the life of the backup.