grur? its guru
there is a way to change it, but you need a jailbroken iphone
install summerboard from installer.app
then, chose a wallpaper from the photo app
in summerboard, scroll down a bit to the 2nd section, and turn wallpaper on
thats the only method i know, youll probably have a few other replies that are better, as theres probably a way to get the two wallpapers different...