I would guess, like Harvey says, you might get away with it. Putting two 12V batteries in series will give you double the internal battery resistance which should effectively limit the current through the starter motor a bit and help to prevent damage. However, you will still be putting significantly more current through the starter than with 12V, so I would avoid long bursts on the starter (to reduce risk of burning out starter and exploding batteries). If it was me, I would play safe and try Chris's 12V batteries in parallel suggestion first.
Guess the other thing to watch out for if the engine is still in the car is all your 12V electrics like bulbs, radio, wiper/heater motor etc. I would make sure it is all switched off (maybe pull the fuses just to be sure?).