Alt-Shift-8 does work in that it splits the window into 2 panes horizontally. After this, create a duplicate view of the current file with File..New View into File. Then drag the new tab to the bottom pane.
I guess the reason for your confusion is that unlike some editors each window/pane has its own buffergroup as defined by the file tabs, which can in turn be freely moved around between panes and windows. Coming from Brief and some other editors this tighter association between files and views can be a little confusing at first, but it's just as flexible in practice.
To your other point about docs, yes, they're less than ideal. But the weakish docs don't really diminish the experience of using Sublime since it's easy to master the basics and fairly intuitive. The lack of docs are however a real PITA if you want to start writing plugins which is where a great deal of Sublime's magic lies.
You might want to check these vids out. They're a great way to get around all the basics of Sublime.