For example, i would travel from Gare du Nord to Mairie de Saint-Ouen. I am using the RER E from Gare Magenta to Gare Haussmann Saint-Lazare, after that i am taking the metro (13) from Saint-Lazare to Mairie de Saint-Ouen. Can i travel like this with one or do i have to validate two t+ tickets?
You would buy a single RER ticket for the entire ride from Gare du Nord to Mairie de Saint-Ouen. The single RER ticket / "billet origin-destination" works on both Paris RER and Metro trains when it is correctly purchased with the origin and destination stations specified. You can buy such tickets from ticket machines or from ticket windows.
Keep the ticket throughout your journey and use it whenever you need to pass through gates / turnstiles.