In a stereoscopic scene, rendered files produced by each camera of the stereo rig are imported and composited in the same composition at all levels (pass, layer and scene). There are two modes in which you import a stereo scene, either in multi-stream mode or not—see The Pre-Compositing Import Options.

In single stream, the graph of the template is duplicated for each camera and placed within a group node to avoid name and expression clashes. The left and right of the output link nodes feed the corresponding dependency graph.

For example, if you double-click the left group, you'll see the default render layer template in the following dependency graph.

In Multi-Stream mode, the left and right outputs of each link node are merged together, feeding only one instance of the dependency graph. At the end, each stream is extracted—see Multi-stream Compositing.