Updated column values without deleting formulas

I have Column A and Column B and I want to have people be able to enter either a value in Column A and it will
update Column B from a vlookup or be able to enter a value in Column B and Column A will be updated. Is this possible? I've been able to do 1 column that will update other columns after a value is entered but not more columns.

Example data: John:1, Mary:2, Sue:3, Tom:4, Chris:5
So, if I enter John in Column A, the number 1 should appear in Column B BUT
then I need to allow for someone to enter 1 in Column B and John to appear in Column A.
This has to be able to go back and forth and not, one or the other.
I'm able to do the first part by having a formula in Column B but if I allow a value to be entered into Column B, the formula
get's overwritten and so then will not allow update the value in Column B if the user then enters a value in Column A of the same row.

I have this working fine updating multiple columns based upon entering a value in 1 column but that's it
=IF(ISNA(VLOOKUP(D7,Roster!A2:D7,2,FALSE)), "", VLOOKUP(D7,Roster!A2:D7,2,FALSE))

Any help would be greatly appreciated as I've pulled out a lot of hair so far and can't afford Rogaine. :-)