Put two apps side by side

1. Show an empty workspace, for example ⌥3.
2. Open the first app. It fills the screen.
3. Put the pointer over the right half of the screen and open the second app. The screen splits, with the new app on the right.
4. To swap them, press ⌃⌥⇧←, or drag one window by its title bar onto the other.
5. To change the split, drag the edge between them.