1. Setup file extensions for session files ⇒ Menu <fc #ff0000>Settings</fc> → <fc #ff0000>Preferences</fc> → <hi #fff200>MISC</hi>

Set Session file ext. to <fc #4682b4>ses</fc> !

1. Open files you want for session (close the rest).
2. Save session ⇒ Menu <fc #ff0000>File</fc> → <fc #ff0000>Save Session</fc> (session_01.ses)

From Command line

"<full path>notepad++.exe" "<full path>session_01.ses" -multiInst

Example:

"C:\Program Files (x86)\Notepad++\notepad++.exe" "C:\Program Files (x86)\Notepad++\session_01_novo.ses" -multiInst

<hi #fff200>“-multiInst”</hi> switch allows notepad++ to open a new instance