I adopted two kittens at the same time. They were not litter mates, however, since they were both young kittens they grew very accustomed to eachother.
They played with eachother and groomed eachother, slept in the same little cat bed, just as real litter mates would. I would consider them adoptive litter mates.
The benefits are they they have eachother; from experience, a single cat will get bored and subsequently tear up more furniture (or otherwise cause a scene). Two cats will play, or argue, with eachother. Furniture is usually left alone with two, I would say.
As per what I feed my cats, it's a 3-way combination:
-Taste of the Wild grain free dry food
-Taste of the Wild grain free canned 'wet' food
-Nature's Variety 'Instinct Raw' frozen raw cat food (I buy the chicken variety because it's cheapest).
These are very high quality cat foods that can only be found at feed stores or 'natural' pet food stores. Don't expect to find this at your local Kroger or Petco. You can also order these brands from Amazon.com.
When I switched my cats to a grain-free "primal cat" diet, my chunky food loving pig of a cat lost 3 pounds. He went from 13 pounds to 10 pounds. He is now very lean. 3 pounds is a LOT for a cat.
Stumbled into Primal due to food allergies, and subsequent elimination of non-primal foods.
Start Gluten-Free/Soy-Free: December 2012; start weight 158lbs, Ladies size 6
Start Primal: March 2013, start weight 150lbs, Ladies size 6
Current: 132lbs, Ladies size 2
26lbs lost since cutting the crap.