I wanted to respond back to this in case anyone was interested.
We went to Dueling Pianos at Ralph Brennan's last Thursday, and it was so much fun! There was no cover charge. Dueling Pianos starts at 9PM, and we were advised by the restaurant to get there about 8:45PM to get a table. When we got there, there were five or six tables left, but they were all gone by 9PM.
We're big Jellyrolls fans at WDW, and my husband actually preferred Ralph Brennan's. It was a really great time, and I hope they continue to offer it. I'd love to do it again on a future trip!