There are split chargers available from In Car enternatment places they ar not cheap. But cheraper than blowing something up or setting fire to things. I have one that wil take anythnig from 6 to 50 volts and tap off at difrent voltages. Maplin's do some very useful kit, including a 24 volt inverter now.
The gadget is made by Sure Power Industries, Tualtin, Oregon Multi battery isolator Mod. 2402. give you a place to start at least.
http://www.surepower.com/