Bob's solution (at post #26) is very creative and certainly lower-cost, involving only one quality regulator and no Centerfielder (the Xantrex EchoCharge is around the same cost as Balmar's Digital DuoCharger, but only passes 15A (vs DDC 30A); and cannot be set up for different battery technologies, meaning whatever charge profile is applied to the House bank is imposed on the Start bank too. I think the DDC is the better kit for that purpose.)
The upside of the (higher cost) 2-regulator + CenterFielder solution is that you get two features not noted in my original post,viz: battery & alternator temperature monitoring.A battery's (or battery bank's) acceptance charge is highly temperature-sensitive and this also varies with the battery technology. So a regulator that can be set for a specific battery type (flooded, gel, etc) and which comes with a battery temperature sensing thermocouple, allows the charging rate to be automatically regulated and optimized over seasons and even during the course of a cruising day, as temps change. This is all about re-charging as quickly as possble consistent with long battery life, so this is a really valuable feature & makes a big difference to re-charge times in my experience.
Alternator temperature sensing is a real favourite of mine: the Balmar gear adjusts the output down of an alternator that is running a bit hot until it has cooled somewhat and then brings it back up to full output when its temperature has stabilized. The % of field adjusted and the temperature set-points are user-programmable. You might be surprised, when you contact an alternator manufacturer and ask them what maximum temperature they are happy for their alternators to operate at for extended periods, at the answers you will get: marine environments are very different to automotive and often with our installations, air flow & cooling of the alternator is nothing like it is on a truck.
The bottom line: different solutions with different costs and levels of sophistication are available, but the practice of directing all twin-engine alternator charge to the House bank in the first instance is a great idea if you want to minimize genset use. In my own case, 4-5 days out on the hook with lots of AC gear& a 3000W inverter (sat TV, dishwasher, electric oven, laptop charging, teenagers & other hi-demand consumers) involves no genset use or solar panel at all, just a move of a couple of hours/day between anchorages.