How can I prevent my 24-month-old and 11-month-old from waking each other up at night?
You can't change a light sleeper into a better sleeper. The only short-term solution I can think of is to have your children sleep in different rooms, widely separated. You could also use "white noise"such as a radio with static or a CD with noises of waves, heartbeats, or windto keep the younger one from being awakened by the older one or get her to sleep faster after she is awakened.
On the other hand, you may be able to improve nighttime chaos with a technique called "scheduled awakening," which can work if the older one wakes up at about the same time every night. Wake her 10 to 15 minutes earlier every night so that her regular awakening gets closer and closer to bedtime.