As the saying goes, two are better than one, and it's true when it comes to kittens. Introduced early and slowly, kittens from two separate litters can form a special bond that will last a lifetime.
Age
Kittens the same age tend to get along better than those with big age differences. If possible, get kittens that were born within a week of each other. Also, the younger kittens from two different litters are when they are introduced, the more accepting they will be of each other. This doesn't mean that kittens of different ages or that are both on the older end of kitten-hood won't get along, but it may take a little more patience and time before they become buddies.
Sex
As long as they're introduced when they're young, the sex of the kittens doesn't matter. Any combination -- two females, two males or one of each -- should get along just fine. Keep in mind that your kittens will need to be spayed and neutered if you get one of each sex or you could end up with unwanted kittens. Once they're adults, two female cats may bond more closely with each other than with their humans, while two male cats tend to act like buddies and in mixed pairs the female is usually dominant.
Introduction
Introduce the kittens slowly, giving each plenty of space. Offer plenty of toys and distractions so the two can play both separately and together. Let them interact with each other while you supervise. They should be allowed to work out any differences on their own, unless there's a risk of injury. If they do fight and need to be separated, throw a blanket over them both, spray water or use a broom to separate them. Don't reach in with your hands. Kittens might be small, but they can do some serious damage with their little teeth and claws.
Space
Giving each kitten his own space will help reduce spats. Offer food in separate bowls and make sure they each have a place to get away to if they get tired of or fed up with the other. Plenty of toys and a large area to play are also a good idea so they can entertain themselves and each other.