My books would get sorted twice as fast, since I wouldn't need to explain the difference between authors and editors, anthologies and novels, series and standalones, how the same author can have different publishers (in other words, all the yellow-spined DAW books do NOT go together on the shelf!), or the difference between fiction and non-fiction (yeah, I actually had to explain that to two college graduates who were "helping" me organize my first apartment).
My cat would be ecstatic because she'd have double the attention and two people to see to her every need.
But my double had better know something about computers.