@Gian-Matteo - Try experimenting with Notepad++'s -settingsDir command line option. If you point it at a folder that is not shared with the other computers then it will be in it’s own session.
Keep in mind that Notepad++ reads its configuration and session files when it starts and writes then back out when you exit or close Notepad++. Thus, you will need to move and think carefully when testing out and using more than one copy of Notepad++ at the same time either on the same or different machines.
It is likely possible* to do what you want to do but you have not provided enough information about how you are currently set up for others to provide suggestions that may be a better fit for your situation or desires.
* I had to add an asterisk as there is one issue you may run into which is Notepad++ does not have perfect support for running two or more instances of Notepad++ on the same machine at the same time.