Complete the story twice, once with the Hero route and a second time as Infamous or vice versa. After the second play through, after the credits end, a message will come up as the post game becomes available that will say that you have completed both paths in Infamous 2 and have unlocked access to the opposite Karma powers of your current Cole along with the powers of the character you didn't transfer with, you are also granted 15,000 XP to help with paying for them for some extra post game fun.