And the grounds of the charger circuits are not common grounds. If they were, it would create a dead short, but they are not. A 24 volt charger could work, but if the batteries are not identical, one will be limiting the other, perhaps severely.You don't charge a 24v bank you charge 2 12v batteries. I have a 4 bank charger hooked up to all 4 of my batteries. Two are hooked up in parallel via a switch and 2 in series for the trolling motor.
Like I said, or Rick said. As for the multi-battery 12V charger, it will work if the chargers have are independent, ie isolated circuits. The first test is to measure for continuity between the ground leads. there should be none. You can solve the problem by unloading that puny GP27 on me at cost. That way you can sleep well. I currently need 2 GP 27 4 my Outrage Project. FYI I typically get them at Costco.