I agree that it would be nice to have some minor way to visually distinguish the two. It could be something as simple as a large crater on one but not the other (which seems scientifically plausible especially for a young system), or even a distinctive lava pattern (an X-shape, a large crescent, or whatever). Other than an aesthetic cue, I like the idea that the two planets are mostly symmetrical for the purpose of orbital maneuvers.
Sure, if the player explores one planet, the other may not add much novelty, but by exploring any one of the pair you'll still get plenty of unique aspects like the tidal lava or seeing the other (very large) planet in the sky.
Another way to think about it: If you were to merge the surface area of Rask and Rusk into a single sphere, would you explore all of it? Probably not. Consider a body like Duna, a player may explore the northern hemisphere and not feel the need to explore the south since it's largely symmetrical. That said, I think having a few easter eggs on each or some unique science biomes (or whatever system KSP2 uses) would be a nice motivation, but this doesn't really require unique visuals.