I was rereading Naruto part II imaging how great it will be animated, when I noticed something:
In volume 29, chapter 258 Naruto uses rasengan by himself. Just look at page 4 where in the first panel we see a panorama view of the battlefield.
Look no clones
Next page we have Naruto using rasengan
And he also isn't kyubified, just plain ol Naruto using rasengan
I think he only used clones in training because it is easier to make the rasengan, but I'm not sure, it might have been a mistake.
I have no proof he can do it with one hand, that is just speculation