There are devices called "charge balancers" for split systems. Then(within reason), you can treat the batteries as independent as long as the appliances electrics aren't grounded. If they are, one lead to earth, and the other to battery/s, 12 or 24
Ideally, silver-solder. Not particularly difficult to do. A small blowlamp silversolder and flux for it. Alternatively, you can get some Lloyds approved marine repair tape. It is moisture curing polyester and glass bandage. Even damp air cures it, but instructions say to wet the bandage before application. .. various widths and lengths. http://www.thistlebond.info/products/engin.../piperepair.htm