My girls are 13 months apart and they also shared a room until they were about 9 and 8. when they were toddlers and had beds side by side, I had a problem with them falling asleep because they would talk and goof around even after I put them down and did our nighttime routine.
What worked for me was....literally sitting on a chair outside of their room and stopping ANY talk that I would hear. It scared them to know I was right outside and that I meant business. I wouldn't leave until they went to sleep.
I gave them about 15 minutes before bed to fool around, but then when it was lights out, I meant business. They knew I would catch their every move so they pretty quickly calmed down and couldn't help but sleep since I wouldn't let them talk. It took about a week or so of doing that, but then they just got in the routine of 15 minutes to play...then it was lights out. If I had problems here and there after, I just warned them that if they didn't they would have something taken away the next day because they wouldn't be able to handle it without sleep...or I reverted to sitting outside their door again....it even works now at 11 and 12 when they have sleepovers and they are up to late with their friends. I stand outside their (my kid & their friends) door and tell them to be quiet...they can't help but to be quiet and settle down because they know I'm right outside the door and won't leave till they are.
hope it helps
T. mom to 3 girls...12,11, and 7.