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They help each other burn off energy.

These kittens are "playfighting," an essential part of their development
Michelle aka michlmc
Even the most devoted human caregiver can quickly become exhausted by trying to keep up with the energy of a single kitten. Two kittens will wear each other out, leaving their human parent free to just enjoy watching them. The downside to this, of course, as Judy Litt so aptly put it is, "...but of course, sometimes you have double trouble, too -- I still remember the time I spent at least 15 minutes getting one or the other of them out of my armoire; everytime I got one out, the other one was in there."