Why would you need two elements? They are both just resistors, granted when running on dc you would produce less heat as the power of the element puts out would be reduced by 64 for a 120v element being heated by a 12 v, only by 16 if using a 24 v dc system. One would only need to isolate the two voltage sources via a relay or an isolation circuit.If the batteries are frequently topped, the idea of a heat sink into the hot water heater is a great idea. Of course, that requires a smart controller too. The real trick is how could you have both the 110v element and the 12v. I think the best the 12v is going to do is get you a good start.