I'd use 2 seaparate batteries to jump a 24V truck....its too easy to burn up a computer or other sensors on a later model 12V vehicle,the diodes they install in the fuse box to prevent damage to the electrical system if someone hooks jumper cables up backwards dont always pop fast enough to prevent expensive things from going up in smoke...on an old vehicle you "might" get away with doing it,but its still risky as hell...
I used to jump start my old '63 VW Beetle that had a 6V battery with a 12V one all the time,even after someone told me it was a potential bomb waiting to go off,they said when you put 12V to a 6V battery,instantly you get an 18V potential dead short.....well,one day when I leaned back to hook up the jumpers I'd left connected to the 6V battery under the seat to the 12V jumper battery behind the passenger seat on the floor,a spark crackled like it always did--then the battery under the seat went off like a grenade,and the acid pretty much destroyed all the interior and made the car rot away quickly,despite me dousing it with baking soda and flushing it off good with a garden hose...
I was dam lucky my eyes didn't get any acid in them....I decided it was about time I just replaced the 6V battery and be done with it...my friends dad showed me how to drill into the case of the 12V battery and tap into the cell connector inside it,to get 6V out of it using 3 out of the 6 cells,and he also rigged it up so I could send 12V just to the starter alone...that thing spun over like never before and always started instantly after I did that modification...