Every person who owns a vehicle with multiple batteries should spend a few minutes or a week if necessary to understand how their batteries are hooked up. That could be a truck, battery, or life, saver.
There are safe shortcuts such as jumping only the dead pair of batteries to get the vehicle going, 12v to 12v.
There's just too much BS information running around about what absolutely won't work, or what will. depending on who you ask. If you don't know, best to not give someone information. That would apply to me too, if I were ever wrong, blah blah blah.
The trick for charging one vehicle off another, 12v to 24v, is to NOT have the vehicle chassis's connected together. Basic electricity.