When rendering out a scene with a stereoscopic camera, Maya renders out the individual left and right camera images. The images have to be composited in order to apply the desired stereo effect.

1. Open the Render View window by selecting Window > Rendering Editors > Render View.
2. In the Render View window select Render > Render > <stereoCamera>

   You have now selected your stereo camera for rendering

3. To view your render output in other stereo modes select Display > Stereo Display