Personally, I wouldn't take the risk. Accidents can occur even at the slowest speed or with the simplest things like slamming on the brakes. For adults, such things can lead to sore necks and backs but they can be deadly for babies. Even if you were to carry one baby, if you get into an accident, the one with the most injuries would be your baby.
I'm even willing to carry my car seat all the way to KL even if it means more trouble for me and my hubby. I would never put my child's life at risk in such instances so for me, better to be safe than sorry.
Is the trip long? If it isn't, try squeezing in or worse come to worse, hire a cab?