Stay with four batteries. No need for a fifth battery - just changes in how they are wired.
Two starting batteries, each wired to a regular 1-Both-2-Off battery switch. One battery to "1" on both switches, and the other battery to "2" on both switches. Each engine is connected as the "Load" on one battery switch. Usual arrangement is one switch in position "1" and the other in position "2", but you can use both to start one engine if needed. Each engine charges its respective battery through the main power lead.
Two other batteries should be a matched set of either Deep Cycle or Dual Purpose, wired in parallel. These go to a simple On-Off battery switch, then feed ALL the house loads through the 40-amp (or larger) circuit breaker that should already be installed. Get the wiring for the Auxiliary Charging Lead and connect BOTH engines to this battery bank - that way either or both engines can charge the house bank. No need for fancy regulators, balancers, isolators, etc.
The Charles Charger installed at the factory on our boat has three bank capability, so it charges both starting batteries and the house bank when on shore power.
I set my system up this way in October 2005 when we repowered from a single 250HP OX66 to the twin F150s. The starting batteries were new at that time, and the two house batteries (West Marine DP 650s) were six months old. I just replaced all four batteries in April - yes, they were that 'old' but still doing pretty well. The starting batteries were fine but the house bank needed replacing, so I did all of them.
My electronics NEVER blink when I start engines, and rarely when we hit the electric flush on the head.
I also have two fixed VHF radios, to separate antenna, with one powered from the house bank and the other from the engine batteries.
Brian
