A two column solution might be:
In column A, your number
In colunm B, their number
in column C
=IF(ISBLANK(A2)," ",VLOOKUP(A2,B$2:B$34,1,FALSE))
in column D
=IF(ISBLANK(B2)," ",VLOOKUP(B2,A$2:A$34,1,FALSE))
change the $34 to be how many rows you are comparing.
The isblank function is there to stop the comparison if the source cell is empty.