Spiralling Serial Spheres

The Twin Rasengan simply refers to the ability to form two Rasengan, one in each hand. When Naruto performed this technique he used two different versions of the Rasengan, the Gelel Rasengan, with the aid of Temujin (who helped form it like a shadow clone would normally), and the Demon Fox Rasengan, with the aid of his shadow clone.

Trivia

 * Jiraiya does something similar when teaching Naruto how to use the Rasengan; in one hand he forms a regular Rasengan, and in the other he does the balloon exercise.