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One kitten can just drive an older cat nuts.

Older cats' patience runs thin when a kitten wants to play
Franny Syufy
Although it might sound contrary, an older, established cat will probably accept two kittens better than one. One kitten will seek out the older cat as a playmate, or worse, tease and pester the senior cat which can cause stress to an older cat. The kitten in return, will be "rewarded" for his playful efforts with hisses and swats. Two kittens will expend their energy in play with each other, leaving their older "uncle" to relax in peace.