Maestro, this is one of those issues with lots of wrinkles and laden with personal preference; only the basics are true science. First, we need more info on use of your house bank. E.g. when the boat is used regularly, what's the boat's typical energy budget inbetween dock plug-ins? E.g. how many amp/hrs do you consume each day, and do you spend nights at anchor or on a mooring vs. being plugged in. Also, knowing you have a 70 amp alternator doesn't say anything about how the battery is charged, since most alternators are not regulated externally and so after only a few moments, a partially depleted battery is only getting a small charge, even from a big alternator. Finally, say a bit about your boat. If you have a 24' overnighter you only daysail, your bank is probably fine the way it is. If you have a 36' sloop you plan to sail on all summer long, the 220 amp/hrs of capacity in one dual 6-volt bank might prove inadequate (especially as you add more systems over time).