No we shouldn't add the resolutions together, because it's not an adding situation.
Let's take wonderful 101 for example. In one scene you're in the cockpit of a plane on the gamepad, controlling a schmup kind of game on the tv. The TV is running at 720p, and the gamepad at 480p. Does this mean that I'm looking at a 1200p picture?
No. I'm either looking at a 720p image or a 480p. You can only look at one screen at a time, so how could you logically add them?
I could see the point if you're talking about the overall horsepower of the Wii U, but if we're talking about how good a game actually looks, it does not make sense to add the numbers.