Resizable custom editor panes side-by-side?

I am making a custom editor that involves selecting things from a tree view and displaying details about them, much like a file browser in Windows. You ever notice how those file browser windows let you drag the border between the tree and the details to resize the two panes? I want to do that.

So, first of all: Is there a way to do this within the UIElement system?

If not, the other obvious approach is to make two separate EditorWindow objects and dock them in the same window. There's a variant of GetWindow that lets you specify a type of EditorWindow to dock the new window next to, but unfortunately, it doesn't do what I want: instead of dividing the space between the new pane and the old one, it puts them into tabs, so that only one of the two windows is visible at once. The user can rearrange them, but is there any way to get it to come up in the desired configuration automatically?

(The Console window is a good example of the effect I want, except that its two sections are arranged vertically rather than horizontally.)