And it's not a double sharp.
I saw it in Miley Cyrus Fugue in measure 88 between the 3rd and 4th beat for the right hand. It's being reached at 2:42-2:43 in the video and I don't hear anything special. Here is a screenshot for convenience:
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May I suggest that it is not an 'x' per se, but actually two lines clarifying the voice leading for the top voices. Such lines are found in the first two bars as well.