Xcode 4 + split screen feature

First, you can change the layout/placement of the Assistant under the View > Assistant Layout menu. This includes placing all assistant windows in the same "column" as the main editor.

Second, you cannot split the main editor. That's what the assistant is for in Xcode 4.

Third, Assistant windows aren't controlled by the main editor unless they're in one of the automatic modes that base their selection upon the main editor's. The only way to navigate them independently is to use their Jump Bar at the top of each assistant panel.

Update

One tip I just remembered: Option-click a file in navigator to open a file in the assistant; Opt-Shift-Click to be prompted in a popup window where to open the new file (double-click an area to choose it or double-click the + sign to add a new area).

A bit late in the game, but here is a cool way to open your files in different placement configurations (e.g. new window split, new tab, etc.):

Key-combination:

Shift + Command + Option + ,

The above allows you to choose between multiple layouts. Click on the + signs to extend the layout shown on the screen.