For reference I've seen all the movies, all of The Clone Wars, Rebels season 1 and up to the episode where Ezra is looking for his parents in Season 2. I also read the Son of Dathomir comic. In all of that material, the only times I remember seeing people on the same side working together to practice dueling was when Dooku was training Savage Opress, and when Opress challenges Maul. I can't remember any other people practicing against each other.
There's an episode in Season 3 of Rebels that shows some training in the manner you're referring to, but I won't spoil any more. I may be mistaken, but I think Ventress and Vos duel to train on Dathomir in Conspirators, the third part of Dark Disciple.
Of course right after I make this thread I find another example. In The Mystery of Chopper Base Kanan and Ezra practice dueling.
