How do I get them to sleep at the same time?

Put your little ones to bed drowsy, but not asleep

Help them learn to fall asleep on their own by letting them drift off once they're in bed, rather than just before you put them down to sleep.
Allowing them to settle themselves to sleep

For the first several days, your children may negatively respond to each other at night. Most likely, your oldest child will go to bed fine, but the crying of the baby will awaken or upset him later on. The process of teaching your oldest to sleep heavier takes some getting used to for him. Reassure him, or if possible, let the baby fall asleep first and then take your oldest to bed.