Hmmm, I'm really not sure. Maybe Shuouma can chime in here. He might know more than me. I just know you need to set DMZ on the main router to the IP of the secondary router and then set DMZ on the secondary router to the IP of the Dreamcast. In essence, you're creating a tunnel through the two routers to your Dreamcast.
Edit: I had the IP of the second router wrong! Set the DMZ on the primary Xfinity router/modem to 10.0.0.16.