In `16.9 Loading Files' of Intro to Emacs Lisp under emacs23.0.96.1 .
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To replace the key binding for the default
`split-window-vertically', you must also unset that key and bind the
keys to `split-window-quietly', like this:

(global-unset-key "\C-x2")
(global-set-key "\C-x2" 'split-window-quietly)

Please note "must" in above paragraph.

Thanks