I have noticed a huge personality variance in the few dwarf gouramis I have had, one was incredibly shy, one was pretty chill, and the one I have now is incredibly mean, and picks on the gold gourami twice his size. I would give each gourami 15-20 gallons of room each.
I have a gold, dwarf, and paradise in a 30, and they chase each other quite a bit, but at night they all hang out together.
As much as people can tell you to expect from a certain fish, there's always that unpredictable individualism.
I would add them at the same time, if you do one then the other the second will be seen as an intruder most likely.