Jerry
I am using 3 Autozone Duralast Dual Purpose Batteries (aaprox $80). I have them wired thru 1 switch from each engine so I can use 3rd battery for starting either engine and charge from either engine. I have generator hooked thru port engine battery switch. I have been thinking of adding a forth although I haven't had any problems.
Duralast Batteries are made by Johnson Controls, same as Die Hards.
http://www.batteryfaq.org
Rob