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Both.
Young cats require a lot of attention and so having a playmate benefits them. It stops them from being lonely (the part where it benefits them) and stops a single cat from being too needy and requiring attention late at night / early in the morning (benefiting us).
Here is an article (patronising as it is) giving a few good reasons: http://www.companionanimalpsychology.com/2012/06/one-kitten-or-two.html?m=1
Finally Celia Hammond also make it a key policy to try to home kittens in pairs whenever possible:
Is the 2nd cat for the cats benefit or yours? Why does working long hours mean 2 cats?
Surely it'd be happier living on its own.