This layout is the same as XMonad.Layout.TwoPane except that it keeps track of the slave window that is alongside the master pane. In other words, it prevents the slave pane from changing after the focus goes back to the master pane.

Import the module in xmonad.hs:

import XMonad.Layout.TwoPanePersistent

Then add the layout to the layoutHook:

myLayout = TwoPanePersistent Nothing (3/100) (1/2) ||| Full ||| etc..
main = xmonad def { layoutHook = myLayout }