You cannot legally separate if you don't live in a state that recognizes legal separation.
If you don't live in one of the following states, you will have to get divorced in order to work out a legally binding way to divide assets, share custody, etc.
Anything in that document should be something that you are prepared to live with, possibly on a permanent basis.
Below is a general chart outlining the main differences between being legally separated and being legally divorced.
Legal separation is sometimes called "Separate Maintenance" or "Divorce a Mensa et Thoro." Being legally separated is more official than being a married couple deciding to simply live apart from each other.