Readers wondering how the split screen feature in El Capitan works, this is for you (otherwise this will be anticlimactic).
Quick Reference:
Step 1:
Select the first window by holding the zoom button and dragging to one side of the screen.
Select the second window for Split-View.
If this doesn't work at first please try the longer directions below (which show a second approach to split view)
Verbose Directions:
Enable "Displays have separate Spaces":
System Preferences
Mission Control Settings
"Displays have separate Spaces"
Select the first window and enable fullscreen via the Focus button:
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Select the second window and enable fullscreen via the Focus button:
In mission control (see below), snap the spaces together: