Well, the fact that they're both lazy short-haired brunettes DOES make them similar, so yeah. XD I'm sure there are more similarities that I just can't name.
Hmm… They're both energetic and have a smile on their faces, but Yui's is more innocent, and Ritsu's is more mischievous.
They seem to be a bit "dim-witted" sometimes (I don't mean it in a bad way) but Ritsu is obviously smarter than Yui and knows when to be serious. So yeah, that's all I could think of. XP