For a start, they tend to be expensive, which is understandable considering the extra hardware but still lamentable.
Then there's driver/game support that can, if not up to scratch, mean some games simply don't work or don't see any benefit from the second chip. Finally, there's the fact that even when everything's working fine, the performance increase from the second chip can be limited.
It's for these reasons that ATI's last attempt at gaining our top graphics card recommendation, in the shape of the HD 3870X2, fell just short of the mark, receiving a good but not quite there, 8/10.
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