A CMSimpleCoAuthors subsite is created like a normal subsite or second language.
The template folder "2author" is included in the CMSimple download. You can copy it and rename it as you want, but please consider the folder naming conventions.
A CoAuthors subsite is always a subfolder of the CMSimpleRoot folder (where CMSimple is installed).
You can find more about creating a subsite in the documentation ยป