"Unless the OS and the app are written to utilize both cores you will see very little performance increase if any at all. This goes the same with your PC at home. Saying a phone has dual core or a computer has quad core just makes it sound like it will be faster."
And...
"If you are running apps or programs designed for multi-core you will see a difference. How many android apps are written for dual core processors?"
Answer
Here's another one of the Android community discussion forums that jackson alluded to. One particularly interesting point of view...
"Unless the OS and the app are written to utilize both cores you will see very little performance increase if any at all. This goes the same with your PC at home. Saying a phone has dual core or a computer has quad core just makes it sound like it will be faster."
And...
"If you are running apps or programs designed for multi-core you will see a difference. How many android apps are written for dual core processors?"