Decimal “XOR” operator
Jelly, 14 bytes

DUz0«9_ṚƊṀƊ€UḌ


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Grid of all single digit pairs

A monadic link taking a list of two integers as its argument and returning an integer. 

Explanation

D               | Decimal digits
 U              | Reverse order of each set of digits
  z0            | Transpose with 0 as filler
          Ɗ€    | For each pair of digits, do the following as a monad:
    «   Ɗ       | - Minimum of the two digits and the following as a monad (vectorises):
     9_         |   - 9 minus the digits
       Ṛ        |   - Reverse the order
         Ṁ      | - Maximum
            U   | Reverse the order of the answer to restore the orignal order of digits
             Ḍ  | Convert back from decimal digits to integer


If a digit matrix is acceptable input/output:

Jelly, 12 bytes

Uz0«9_ṚƊṀƊ€U


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