"Twin Sudoku" consists of a pair of standard sudoku puzzles each with some starting digits. To get a complete solution for the twin puzzles, it is necessary to solve each twin puzzle, but the complete solution of the twin puzzles will be obtained by substituting the corresponding values of the digits from one twin sudoku into the other.

What exactly do they mean by "substituting the corresponding values of the digits"? What/where are the "corresponding values"?

Here are the two sets of puzzles:

#53

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`.............6......3.8.5.....3.2....28.9.47....8.5.....4.3.2......2.............`

.6.4.2.3.7.......6..1.6.5..5.......9..6.3.7..2.......1..7.1.8..9.......3.5.7.3.1.

#54

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`...573....3.....4....1.4...5.4.1.2.98..957..63.6.8.5.7...7.9....8.....9....845...`

..........4..5..9...7.1.5............31.2.94............3.4.6...7..6..5..........