Dueling coasters are challanging yet rewarding. Right now, I think dueling coasters are the best place to get creative with your coaster design, becuase alot of people make a coaster, and basically mirror it for the other side of it. I have found that having 2 unique tracks gets high adrenaline ratings. But back to the topic at hand.
When making your track, keep in mind how long a train will spend on that segment. It helps if you get a stop watch or anything that tells times to see how long it takes the train on each side to do something. Think ahead! When you build a segment think, "What would be cool for the other train to do right here to interact with this one?"
I hope I was helpful, to help give you a visual aid check out my Phase Two coaster
http://www.coastercrazy.com/track_exchang ... sp?tid=452
It has many great elements of a good dueling coaster.