It would be weird to have both Nygma and Dent do the Gollum/Smeagol routine. But I think with Dent (if the show ever gets back to him) it will be less two dueling personalities and more sudden bipolar shifts from Jekyll to Hyde.
The final Riddler will probably be a combination of the two personalities... much to his frustration. He'll try to be the confident, decisive and sociopathic guy in the mirror, but held back by his awkwardness, his fascination with riddles, and his reluctance to make trouble.