There is often no real difference between 'could have' and 'might have',
In your two sentences, there is perhaps no difference, though it is impossible to say without further context. In the following two sentences, there is a clear difference.
I don't know where John is. He might have gone to the movies, or he might have gone to the pub.
John was free yesterday evening, so he could have gone to the movies if he had wanted to, but he said he was too tired.
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