@Klaus25,

When you first set up a device with multiple outlets, both of the outlets are automatically added to the app - it may just have a generic name like 'Smart Plug' or 'Plug 2' 

You can quickly navigate to the 2nd outlet's settings page by going to your existing device and swiping right or left to switch between each outlet on your TP25.

If you are looking to add a completely separate device/plug from your TP25, you would do this from the home screen of the Tapo app, with the (+) icon in the top right