I'll assume each battery is dedicated to start each engine and each is available for house loads as selected on a 1-ALL-2-OFF switch. If you're just running between marinas and never need or want a large house battery to isolate your start capability from your house loads, I think 2 G31s will do fine. As stated above, each is capable of starting the engines.
If you don't have long-term or high demand house loads on at anchor (and I assume you have no generator), there's no need for a large house bank. Besides, once you build a large capacity house bank, a whole list of other requirements kick in like larger chargers and alternator(s), large cabling, generators/solar/wind power, an inverter to run all those new AC powered gadgets, etc. It's a long list of improvements needed to make a large house bank useful away from a dock.
And if you have the capability, you might as well add all kinds of new tricks and gadgets to make use of all that power like an extra fridge, ice makers, powerful air horns and washdown pumps, HDTV, Satellite TV, stereos with huge amplifiers, new lighting throughout, microwave, Keurig coffee maker, ...the list is endless!
Enjoy it with just 2 $100 batteries and consider yourself lucky!! You'll save yourself countless thousands!!