I remember watching this video back in 2006. Now they released a rare recording of Rachmaninoff playing most of this dance on 1 piano while humming with it loudly to compensate for the orchestra.
It's not an arraingment. It was orignially written for two pianos. It was played for the first time with Rachmaninoff on one piano and Vladimr Horowitz on the other piano. Rachmaninoff added the orchestration at a later time.
No, and actually this is not an arraignment. For this section it is possible to play both piano parts simultaneously. The fingering gets a little tricky however.