Interesting thought. Guess from my military experience, I've never had to jump off a 24 volt system when I didn't have another 24 volt vehicle around. We just hauled out the heavy cables with the huge plugs, and just plugged grill to grill with them.
Suppose you could hook the 12 volt cables across just one battery, or two batteries what were plugged in parallel on a four battry system, and add the boost from the 12 volt system into half of the 24 volt ststem. Suppose you could even use two sets of cables and jump to both 12 volt sides of the 24 volt system, but that would tricky for someone who doesn't fully understand series-parallel wiring.
Be interesting to see if anyone has actually had to do this. Might even be fun to stand back and watch all the loudish poppin undt spitzensparken.