They are joined at the hip, figuratively. Everything that one does, the other has to do. Eating, sleeping, playing.. everything. (Almost everything... They quickly learned that the litter box is one cat at a time only.)
It's almost like owning just one cat, just with double the poo.
I genuinely believe a second cat is the greatest thing you can have for a cat (as long as they get along good).
If you have the space I'd get a second box in the future.
Depends on the cat. Surely true for a bonded pair like this, but I had one cat who for the remaining ~10 years of her life never got past the grudging acceptance stage when we adopted a second, then a third. That second cat reacted similarly when another cat joined the household years later.
We adopted a brother and sister cat and it's well worth it in my opinion, they keep each other entertained. They also love each other more than I thought was possible lol.