Hi! T.,
We have 4 children, the oldest two at 20 1/2 months apart. Have you thought about making one room for sleeping (for both girls) and the other a toy room. That way the mess is only in one room and older one is not put out of her room while the baby is sleeping.
Please remember that even though they are very close in age, they are miles apart in developement. When our 2nd daughter was about 3 she was just one size smaller than her sister and people often asked if they were twins. We forgot for awhile that Timarie was almost 2 years younger and we were expecting her to act as her sister. Then one day when S. was in first grade and Timarie still in pre-school, it dawned on me that there was a really big differnece in their abilities.
Also, remember that which bedroom is whose is not a big deal. What is going to work out best for you as their mother. What will make your life work the best. It will make you a better mother because your life will be better. I also believe that it will be important to allow the older daughter to do things first and the younger to wait her turn. You might want to write at what age the oldest was allowed to do things or also required to do things (i.e.,take dishes to the sink, etc). I know that you think that you will remember, and I did for years, however the time will come when you don't. Even if all you do is write it on a calendar.
Now I am way off subject. Good luck and be happy. It is good for them to see a happy Mom.