I’m not sure but they and Intervolt do dcdc battery to battery chargers for leisure batteries. If you were to fit the Intervolt, you could at lease charge one battery at the correct voltage from your alternator and charge both from some solar cells. I’m not sure it would be right though. Might be worth talking to Intervolt as they are an Australian company and very helpful. There may be other ways of doing it. There are of course small cheap trickle chargers with solar panels. Not sure again if they would be what you are after.