L.C.
My 5yr old twin girls are in the same KDG class, and I may keep them together next year as well. They share friends, and also have their own individual friends in their class.
My thought is as long as they do not affect each other socially or academically I will let them decide if they want to stay together.
In preschool they were in the same class, and quite frankly they each did there own thing, they were not attached at the hip.
For me it will make it easier as homework will be the same, and since I work f/t that is a lifesaver.
YOu could alway try them together, and see how it goes.