Oh my god I love this idea. It's the perfect "Hero" version of the Mastermind.
A few thoughts:
1. I think the Martial Assault set is the iconic Duo primary, but the others seem... well. I dunno. I think the image one envisions with this archetype is the Silver Age hero who has a sidekick that has similar abilities; Batman and Robin, Green Arrow and Speedy, Aquaman and Aqualad, Flash and Kid Flash, so on and so forth. A hero being followed around by a golem, a frost elemental or an id beast... these seem not to fit in with that mold. It fits more with the Mastermind, who has variously themed henchmen, but it strikes me that the idea of the Duo seems to more suggest and call back to instances in comic lore where there was an inseparable partnership between two heroes, rather than a hero and some kind of monster-thing. May I suggest some alternate names for the partners from the various power sets you've listed?
Martial Assault: Pupil (perfection!)
Dark Assault: Apprentice
Earth Assault: Geomancer
Fiery Assault: Pyrokinetic (this one is perfect as is)
Icy Assault: Cryokinetic (following the pattern)
Psionic Assault: Neophyte
2. I dunno about you, but I like power sets that give you weapons. Masterminds get access to some ranged weapons. So... how about some love for the Duos with a set that lets them fight with swords, axes and maces?
And how about the other elemental types, apart from just fire, ice and earth?
Weapon Mastery: Duelist
Wind Assault: Aeromancer
Lightning Assault: Electrokinetic
Water Assault: Hydromancer
3. Of course this raises the question that, if memory serves, has been floating around the community since well before NCSoft shuttered the game: when are we gonna be able to customize the looks of our Mastermind Henchmen/Duo Partners?