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blokhead
A pretty neat and simple method is one outlined by [Zaxo] in [id://273638]. Basically, you measure how much it helps a compression algorithm to concatenate the two files together, compared to when you compress them independently.
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If you think of a compression algorithm as a mild approximation to [http://en.wikipedia.org/wiki/Information_theory|Shannon entropy], then this approach is essentially computing the corresponding approximation of
[http://en.wikipedia.org/wiki/Mutual_information|mutual information] (normalized to the range 0.5 - 1), which is the intuitive concept you seem to be looking for.
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blokhead
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