According to me, single handed dual stance would be more like Niman (which no one has ever seen in fight though) because it looks like how some of the Jedi (Coleman Trebor) hold their saber.
As for Ataru, it depends which Ataru we are talking about, everyone refers to Yoda's, but it's not really the true Ataru, because he uses all the Ataru power to jump because he is too short otherwise.
To me, Ataru is how Qui-Gon and Obi-Wan fight in episode 1, especially Qui-Gon.
Fast and strong swings, but slow transitions (force assisted / empowered attacks).
I don't think any stance animation looks like Ataru yet, maybe yellow is the closest though, or red...
By the way, to be honest I don't like the new Soresu stance look in defense (idle) mode... because Obi Wan just holds it like that to provoke his enemies, then when it comes to fight, he doesn't point his hand at the enemy anymore and has his saber in front of him.