Data List - Write to another cell with check

[magix]

Let say in excel cell A1 and B1, I created a List using Data Validation, and
put in value 1,2,3,N/A

So that cell A1 and B1 each will have drop down list 1,2,3, N/A

Now, How can I achieve following with formula:

- If in cell A1, after I select value 2 for example, it will check if B1 has
select the same value 3,

Pseudocode is like this:
If B1 is empty or B1 has N/A,
write value 2 that selected into Cell C1

else If B1 has the same value as A1( in this example: 2)
it will prompt error, saying that value 2 has been
selected in another cell.and user has to select other value in cell A1

else
Compare the value with cell B1.
if B1 value < A1 Value
write B1 value into Cell C1
else
write A1 value (which is 2) into Cell C1
end if
end if

same check goes to Cell B1


Please help.

 

[magix]

To add on,
If in Cell A1, if I select N/A, no check will be done.

Same goes to Cell B1