Layouts that splits the screen either horizontally or vertically and shows two windows. The first window is always the master window, and the other is either the currently focused window or the second window in layout order.

You can use this module with the following in your xmonad.hs:

import XMonad.Layout.DragPane

Then edit your layoutHook by adding the DragPane layout:

myLayout = dragPane Horizontal 0.1 0.5 ||| Full ||| etc..
main = xmonad def { layoutHook = myLayout }

For more detailed instructions on editing the layoutHook see the tutorial and XMonad.Doc.Extending.