It pretty much depends on how you use the batteries, i.e., do you typically draw them down 50% or more such that it is possible you may be without power to start the engine? How do you feel about using a battery selector switch and the need to ensure it is always on the correct battery? Do you want to spend the extra cash to install a separate start battery, wiring and deal with it's charging?
The main advantage (of a separate start battery (or as some prefer to cal it an "emergency" battery) is that you can always start the engine regardless of what the house bank capacity is. If,however, you find two 8Ds at over 400 amps way too much capacity for your expected need, you might consider separating them and forget about the start battery. Or, you can go with the two 8Ds and use them as both a house bank and starting battery if you are confident you would never deplete them.
Personally, I find your initial inclination (to go with the start battery) to be the ideal situation and use the two 8Ds as one large house bank but my electrical needs may differ drastically from yours so one solution doesn't fit all.
I know I didn't answer your question but frankly there is no single best answer, just things to consider.