1. Add folders to project
2. Save it (Project -> Save Project As…)
3. Duplicate project files and rename the project
Though .sublime-project will be enough to copy, .sublime-workspace contains some additional preferences on history, unfolded directories and others.
4. Open duplicated project in a new window
- Go to folder with the .sublime-project file
- Open the file Properties
- Press Change and select Sublime Text .exe file
- Doubleclick the .sublime-project file.
Through Sublime Text
- Go to Project -> Open Project
- Navigate to the duplicated project file and open it.
- Use Edit file feature in such programs as Total Commander or Double Commander, the key is F4.