When two, or more, people work on the same website in Simply Edit and they try to save their changes, then only the first change will go through.
For example, when Foo is editing Page A and Bar is editing Page B and Foo saves their changes first, then the changes of Bar won't go through.
Bar will get a Is someone else editing? message, and that should have implied that the changes will merge with one another. However that does not happen and only the changes of Foo will go through.