You can do it without any root. You just have to edit the apk in order to make it work with multiwindow.. There was an app here on xda that modifies the apk, so you only need to download the apk from whatsapp site (not the market), edit it, and in...
you should find it here: http://forum.xda-developers.com/showthread.php?t=1740061
the modem LH4 can be found here: