How to remove a value (from the adjacent cell) in the drop-down list?

Question

I have two columns Product 1 and Product 2, both of which lets user select from a dropdown list. But I don't want to show the same value in Product 2 if the user selected it for Product 1 in the same row.

The formula I am using for the drop down list is:

='Details'!$C$2:$C$500

Answer 1

If helper columns are allowed drop this formula into D2 and drag down:

=IF(C2=PRODUCT1,"",C2)

I don't know what cell the first dropdown is, just plug that cell ref in where I said PRODUCT1, make sure to use $ to lock the cell ref. (ex. A1 become $A$1)

Then select your second drop down and set its data validation to ='Details':$D$2:$D$500

It will leave a blank where the value used to be.