Two dissimilar things is Whitman comparing when he uses the metaphor of a little house:
The poet Walt Whiteman has talked about the perfumes or smell of anything like a metaphor. He has related the sense of smell with memories. He has talked about the smell of a house or a room. Also, at the same time, he has talked about the smell of books on the shelves.
The two dissimilar things he has mentioned were specific about the house memories. One is the smell of the house or the room and the other is the smell of the books on the shelves. All the above two things remind us about our past memories.
