I wouldn't add more fish. Adding more fish never solves aggression problems when dealing with solitary fish (schooling fish will sometimes calm down when with lots of their own kind...). The more cramped they are, the more they'll fight. I have had gouramis before, and they always fight. They have to establish a heirarchy. Once that's done, all will be fine (or you'll have one less fish...). If the dominant fish is really beating up the other one, causing physical damage, then I suggest moving the weaker fish out. Otherwise, this sounds pretty normal to me....I agree with adding plants for cover, both floating and buried.
Then again, mine were in an aggressive cichlid tank, so I could be wrong when talking about JUST gouramis...