I thought this was an interesting site, it suggests that the use of two wah pedals in series can immitate the formants of the human voice more closely than using one pedal. Personal experimentation has demonstrated that where there are only 3 distinct tonal positions (by this I mean clear discontinities in the tone) on my wah when I have only one pedal in use this increases to 4 when I have two in series with one of the pedals set on high and the other varied.
If anyone has any advice or observations on the use of the wah pedal (single or otherwise) I'd be interested to read about them! Have fun!
http://www.geofex.com/Article_Folders/wahpedl/voicewah.htm