I wouldn't suggest going to DUS for a conference, then going to Mainz for a night, and then returning to DUS to fly back. If you were flying out of FRA, Mainz would be a great place to spend your last night.

If you have a specific reason to go to Mainz and don't have an early AM departure from DUS, then it would be a possibility. Trains from Mainz to DUS will take 2.5-3 hours.

I'd recommend staying somewhere within the VRR (Verkehrsverbund Rhein-Ruhr--regional transit network)--Dortmund, Essen, Bochum, Duisburg, etc. In fact, you could do this during the event itself, as the trade show passes in DUS (Euroshop included--I just checked) include free transit to the show and back within the VRR regional transit network on the days of the event.