You can easily switch between Sidekick and other presentation software (ProPresenter, PowerPoint, Keynote, etc.).
Each app sits in a layer on your presentation screen. All you have to do is turn off the layer(s) above the layer you want to see.
ProPresenter uses the "Output" button to turn itself on/off. PowerPoint uses the "exit presentation" button.
Sidekick uses a display modes "Live/Off", You can find this by clicking on the TV in the upper-right corner of Sidekick, and switching the display mode to "Live" to be on your second screen, or ""Off" to be off, letting your other app take control.