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

 import XMonad.Layout.Combo

and add something like

 combineTwo (TwoPane 0.03 0.5) (tabbed shrinkText defaultTConf) (tabbed shrinkText defaultTConf)

to your layouts.

For more detailed instructions on editing the layoutHook see:

XMonad.Doc.Extending#Editing_the_layout_hook

combineTwo is a new simple layout combinator. It allows the combination of two layouts using a third to split the screen between the two, but has the advantage of allowing you to dynamically adjust the layout, in terms of the number of windows in each sublayout. To do this, use XMonad.Layout.WindowNavigation, and add the following key bindings (or something similar):

    , ((modMask x .|. controlMask .|. shiftMask, xK_Right), sendMessage $ Move R)
    , ((modMask x .|. controlMask .|. shiftMask, xK_Left ), sendMessage $ Move L)
    , ((modMask x .|. controlMask .|. shiftMask, xK_Up   ), sendMessage $ Move U)
    , ((modMask x .|. controlMask .|. shiftMask, xK_Down ), sendMessage $ Move D)

For detailed instruction on editing the key binding see XMonad.Doc.Extending#Editing_key_bindings.

These bindings will move a window into the sublayout that is up/down/left/right of its current position. Note that there is some weirdness in combineTwo, in that the mod-tab focus order is not very closely related to the layout order. This is because we're forced to keep track of the window positions separately, and this is ugly. If you don't like this, lobby for hierarchical stacks in core xmonad or go reimplement the core of xmonad yourself.