What if you need to look at both views to play the game. They wouldn't be equally useful then, would they? I was thinking about a space shooter with two views, one from the side and the other from the top, so the gameplay would be 3d but the graphics 2d. Would that be acceptable?

Well I think the equally useful thing means that both views need to be actively used by a player in order to play a game (e.g. a little mini-map, radar, stat-screen etc would not be sufficient). Your idea sounds like it'd be fine as a player would need to look at both views in order to play the game properly.