Open the Shot Editor. Select source for Channel A. Select source for Channel B.
I'm not quite sure where the confusion is from your post.
You can have up to seven layers in the shot editor. They can be anything from video sources, audio sources, stills, video files. Composite them any way you'd like.
There are five Master Layers as well. Each can contain a completely composited Shot with seven layers. That can give you a total of 35 layers.