How to dock multiple screens in a single screen?
The dock has two sides. One side of the dock is for running and not-running Apps. The other side holds the trash can and all folders, documents and minimized application windows.

I've uploaded some retina sized images with an obscenely large hello arrow to show the dividing line.

When your dock is on the bottom, look for this dark thin line.



When your dock is on a side, look for a light set of bars.