I agree wholeheartedly with erasmus.

Also, I have two kids an have never seen a 11kg child who will "suck and sleep" for 12 hours straight. My kids are very large for their ages. My daughter is almost 11kg at 10m, and even she won' so that, so an older child certainly won't.

If two C seats aren't in your budet, then it's time to give up on the C seats and buy two Ys instead.