Funny thing, I get questions from time to time about batteries. My boat has one factory battery and one factory alternator, hopefully it will stay that way. The way I use my boat is start it and go riding, turn it off and put it away. If I can't keep things running well off the single battery I might look into a pair of golf cart 6v batts but I would first need to look into the charging/discharging rates.
In addition to being the king of Bling, Grant has lot's of experience with battery banks. I think the key-on both batteries, key-off single battery approach with a solenoid is a good solution for small systems, it was discussed about a month ago. For larger systems like Jude's X9, Harold's, or Grants, you are going to need 5+ deep cycle batteries running in parallel to push the current demand for any length of time.
Harald, the marketing slogan should probably really be "lose-lose" as it usually describes significant compromizes between two competing needs. Understanding how the stereo will be used is as important as understanding how a 4 wheel drive vehicle will be used. Make the wrong assumption and things just don't work very well.