A layout that splits the screen horizontally and shows two windows. The left window is always the master window, and the right 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.TwoPane

Then edit your layoutHook by adding the TwoPane layout:

myLayout = TwoPane (3/100) (1/2)  ||| Full ||| etc..
main = xmonad def { layoutHook = myLayout }

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