I would probably count multiple actions for one individual as being roughly equal with multiple characters acting in concert. So instead of 2 normally fast opponents, you get one opponent so fast, that he can act twice. It's sort of an application of the fractal, each of his arms is his own character, sort of.
For a player, I would not allow it as a blanket power, unless every player can have 2 characters/actions. If you give everyone 2 actions, that wouldn't be that much different from how it is done now.
I could, however, see a stunt where the character would be allowed, once per scene, to take his next exchange action right now, so he can act twice in one exchange, but not at all in the next. Great for finishing blows, but not too overpowered.