Hi maxcatpix,

You are right; there is only one DVI port at the rear of the Computer Tower for Dell XPS 8500 Desktop. If you are willing to connect the two monitors using HDMI ports only, you can use and HDMI to DVI Adapter or a cable for this purpose (please see the images below):

http://sound-affair.com/image/cache/data/Cables_and_Parts/Screen%20Shot%202012-04-25%20at%2012.02.37-900x900.png

http://sound-affair.com/image/cache/data/Cables_and_Parts/Screen%20Shot%202012-04-25%20at%2012.02.54-900x900.png

http://upload.wikimedia.org/wikipedia/commons/4/40/DVI-HDMI-Adapter.jpg

http://www.mac-pro.com/HDMI-to-DVI-HDTV-Digital-AV-Cable-10Ft-image.jpg

Hope this helps.