I don't believe you can do that, even though no where in the Rev. Rule book does it say that since in that it just says you can only use Assualt once per game the guy from FFG states it can only be used once per round?
"Lead developer Mack Martin comments on the army build options the Commissars afford players:
One of the combinations that will be readily apparent to many SSU players, is the addition of a Commissar with Assault, like The Fonvinzin, to a unit that already has Assault. Not only is it a great way to get rapid units into position, but if the Commissar uses his first, he can be safely removed as a casualty. While a player can't activate both in the same turn, the ability to use Assault twice in a game gives a unit a unique tactical advantage."