Create a Constraint such that only one of two fields must be filled
You need to use a check constraint:

create table kevin
(
  one   integer, 
  other integer,
  constraint only_one_value 
        check (        (one is null or other is null) 
               and not (one is null and other is null) )
);


This ensures that at least one of the columns has a value and that not both have a value. 

If those are varchar columns you might want to check for empty values ('') as well. To do that use nullif() which converts a value to null if it is equal to the second argument of the function.

create table kevin
(
  one   integer, 
  other integer,
  constraint only_one_value 
        check (        (nullif(one,'') is null or nullif(other,'') is null) 
               and not (nullif(one,'') is null and nullif(other,'') is null) )
);