A few months ago, Welmoed from the blog, There She Sews, http://thereshesews.blogspot.com/2010/02/my-newest-garment-sewing-client.html, wrote me to tell me she had put a separating zipper in the top of her Professional Tote. She very kindly wrote up directions and pictures of how she did that and you can find that info here http://thereshesews.blogspot.com/2009/09/professional-tote-zipper-panel.html
The pictures above are one of Welmoeds Professional Tote diaper bags that she made, complete with changing mat. The separating zipper shows up nicely in the bottom picture.
Thank you, Welmoed for taking the time to write up those directions for us!