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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:
Soul-
As some who have good memory would remember I thought about have a 2nd cat over and over again when Patch was still kitten-ish... My only concern is that what if that 2nd cat doesn't get on with Patch? or in your case, your kitten? Who would you give up then? And woud it then be fair for that cat to have to go back to CHAT or whatever home there might be?
I am not going to doubt or disagree what CHAT believe as a charity, but I am always under the impression that charity pairs cats after they believe that can get on and will have tried and tested the pairing before rehoming them.
Do what you feel it's best, but it would pain me to see you coming back here in a few weeks time to say the 2nd cat doesn't get on with your kitten and you are left in an impossible situation.
ExTra
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.