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Well, maybe somebody noticed the same or somebody has an idea why it happened to me.
Let's say both of your sites link to site C and site C has been seen as doing some linking scheme Google does not like - this might explain it.
Let's say your sites have incoming links from site D and site D got a penalty and will not pass PR anymore - this might explain it.
There are many possible reasons for what you are experiencing.
What do the two sites have in common (in terms of linking...)?
So I guess you are not alone, which could make you feel better (did for us:)