IMO, an Echo~Charge device is the best solution for maintaining a separate starting battery. Echo~Charge is a product name used by the manufacturer, Xantrex. There are other such devices by other manufacturers which are comparable.
I've used an Echo~Charge for two years now and love it. Here's why it makes sense.
A starting battery is/should be maintained in a fully charged condition. When you start the engine, the starter draws a lot of amperage (sometimes 250A or so) but for a VERY SHORT TIME. My diesel
starts with an average of less than 5 seconds starter time. Even if you use your starter for, say, 60 seconds @ 250A draw, that's only a total draw on your starting battery of 4.2AH. Mine is closer to a half amp-hour because the diesel
starts easily every time.
So, you don't need a lot of charging to replace that small quantity of amp-hours. The Echo~Charge is a small, relatively inexpensive device which senses whenever there is a charging voltage (above about 12.8V) on your house batteries and, when there is, it will begin charging your starting battery with a current
up to 15A. It will not overcharge your battery and, when there's no longer a charging voltage on the house batteries, it will automatically stop drawing current. Simple, clean, effective, relatively little RFI, relatively inexpensive, and maintenance
free. What more could you hope for?
Thus the "preferred" setup, in my opinion and experience, is to connect all charging sources directly to your HOUSE batteries, and let the little Echo~Charge do its important but relatively easy job of maintaining a full charge on your separate starting battery.