"More Power!"
If you read articles about batteries on Maine Sail's well written site, he advises against using the combining function. His take on it is that if your primary battery does not have enough reserve to easily start our (typically very small) diesels you have other problems to diagnose.
In pondering all this info, I often wonder if the whole DC industry is always having "historical engineering flashbacks" to our grandfather's and father's automotive and boating era when everything was six volts. In the six volt era electrical problems were very common.
One of the huge advantages to the industry move to 12 volts was majorly overcoming the limitations of wiring resistance and typically poor connections. In our marine atmosphere where it's worse, we always have the problem of corrosion in the connections, and unless you have all-tinned wire and perfect crimps in all terminations there will be losses that add up there, as well.
So the vendor's switch advice quoted above is not so much "wrong" as perhaps not really helpful, and it has historical narrative comfort associated with it.
Gotta say.... EY equipped our boat with the same old distribution scheme: a heavy duty 1-2-all switch and a pair of group 24 batteries to use separately or combined. This really was not (IMHO) near enough for more than occasional overnighting.
As an example, there was no "stock" refrigeration upgrade for our model, but if there had been one they would have faced some customer wrath over the low DC reserve. I only cite that because the fridge is one of those features that really makes a boat this size "cruise-able".
Long ago we copied the upgrade on another EY boat in our marina and went to a separate emergency/reserve battery. The stock location would then hold a pair of golf cart batteries for a decent-size house bank.. actually an Everything Bank. I know of EY owners that have achieved this by finding location for an additional 12 volt battery so that they could parallel two of their 12 volt batteries for a similar-size house bank.