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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Pyth, 31 bytes

LhS,hb-9ebjkmeS,ydy_d_.t_MjRTQ0

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LhS,hb-9eb             # Helper function, computes the (x & ~y) part
L                      # y = lambda b:
  S                    #               sorted(                )  
   ,                   #                       [    ,        ]
    hb                 #                        b[0]
      -9eb             #                              9-b[-1]
 h                     #                                       [0] # sorted(...)[0] = minimum

jkmeS,ydy_d_.t_MjRTQ0  # Main program (example input Q: [123, 45])
                jRTQ   # convert each input to a list of digits -> [[1,2,3],[4,5]]
              _M       # reverse each -> [[3,2,1],[5,4]]
            .t      0  # transpose, padding right with 0 -> [[3,5],[2,4],[1,0]]
           _           # reverse -> [[1,0],[2,4],[3,5]]
  m                    # map that over lambda d:
    S,                 #   sorted([    ,           ])
      yd               #           y(d)
        y_d            #                 y(d[::-1])         # reversed
   e                   #                             [-1]   # sorted(...)[-1] = maximum
jk                     # ''.join( ^^^ )

Python 2, 71 bytes

f=lambda n,m,T=10:n+m and max(min(n%T,~m%T),min(m%T,~n%T))+f(n/T,m/T)*T

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