Choose a viewer to always preview
Designating a viewer as the default panel to preview is particularly useful when you have a Composition viewer that represents your final output, and you always want to preview that viewer even when you’re changing settings in other panels.
The panel that’s set to always preview appears frontmost for the duration of the preview.
Click the Always Preview This View button 
in
the bottom left corner of the panel.
Note: When multiple views are open, previews use the frontmost
composition view for 2D compositions and the Active Camera view
for 3D compositions. To turn off the Active Camera, deselect Previews
Favor Active Camera in the Time Controls menu.