Adding multiple sensors is quite easy. Once you have your first sensor installed, simply select the "+" icon on the "Sensors" screen in the SimpleSENCE app.
For iPhone and iPad users, this button is located in the upper righthand corner of the "Sensors" screen in the mobile app.
For Android users, this button will be located in the bottom righthand corner of the "Sensors" screen.
Then repeat the same set-up steps as the first sensor for each additional sensor you want to add.