While I only have one cat myself, I would say go ahead and take both of them. Especially since they are litter mates, they already know each other. They should be good buddies and keep each other company.
One litter box is probably going to be OK... 2 might be preferable, but not necessary. Cats are pretty versatile. They can be be perfectly happy being a loner, but they can also function well with other cats. Since these two are litter mates, they should be great.
The problem I ran into trying to have more than one cat was I have a little Siamese cat and she has been a loner since she was just a baby. We brought another cat into the house (an older cat, came from some in-laws long story). My little Siamese cat terrorized him. She would whip is behind several times a day, every day. He was going "outside the box" as well. The whole thing was a disaster. I was soooo worried that he was going to get tired of the way she would beat him up on a daily basis and really hurt her. She was/is just a tiny little thing and he was quite large.
We found a new home for him quickly.