It's a VERY common battery architecture for U.S.-built production powerboats. In fact, I suspect it's more common than having separate start and house banks.
We're on our fourth U.S.-built production boat brand, all 4 have been set up this way.
And then these can be augmented by separate batteries for the genset (most common), or a thruster, or a windlass.
And making changes can sometimes be easy... or not. (In our cases, not.) Depending on real estate, I've found it easiest to just add capacity to each main bank if possible... or at least ensure capacity is as much as real estate will allow... and then get on with life.
Given a separate starting battery for genset, plus a typical parallel ("emergency") start switch to temporarily combine both main banks if necessary... there's not much chance of being stranded/QUOTE]
This is my experience also with 2x12v AGM 8D’s. Before inverter use became widespread, most boats w/o generators had limited 12v dc loads, as usual the fridge being the most voracious. I recall routinely switching to “Battery 1” on odd numbered nights at anchor, preserving #2 for starting. On even numbered nights at anchor used only #2. The idea was to always leave a battery unused for house loads & even out battery usage so that both were equally dependable. We didn’t have battery monitors and the storage was only effective assuming one ran the mains each day.
Mandatory to know your battery selector switch was a “make then break” type with this strategy but it was simple, effective and safe because one was always using a single (but different each day) battery to start engines, so you could hear and see any decline in performance before it became critical. In my case, Batteries were still fine after 5 yrs when boats sold.