If in your group everyone is a die hard Carcassonne fan and they have time, I guess a double set game wouldn't be so bad.
In my experience, though, after 1 hour of playing most people just want the game to be over already, and if there's time, play a different game afterwards like Catan or Stone Age. In the rare case that people want to play a more competitive and longer game, they ask me to add a few expansions. Each extra 6 tiles expansion adds around 15 minutes to the game session.