I am trying to create an excel spreadsheet where the users can enter pricing information for two different products.

The challenge is I want to link pricing information cells in a way that if they enter the value for one of the products the other value becomes "0" automatically and is locked for any additional data entry.

So, if the value in a1>0 (if the pricing info entered is more than 0) then cell b1 is 0 automatically is locked for data entry and vice versa.

Nothing seems to work so far and I am not sure if this is even possible.
Appreciate any quick help you can provide.