The Spiralling Serial Spheres is a double-handed version of the regular Rasengan.
The user creates two Rasengan, one in each of their hands. The user can then either attack two targets at once, or attack the same target with both Rasengan.
When in Sage Mode, Naruto can use a senjutsu-enhanced version known as Sage Technique: Spiralling Serial Spheres (仙術・螺旋連丸, Senjutsu: Rasenrengan, English TV: Sage Jutsu: Rasengan Barrage). It is fundamentally the same as the normal version, except stronger.
Jiraiya does something similar when he teaches Naruto how to use the Rasengan; in one hand he forms a regular Rasengan, and in the other he does the balloon exercise.