Try this, create a calculated field as below to
IF ISNULL([Field1]) OR ISNULL([Field2]) THEN "One or Both null statements" ELSE " Not NULL statements" END.
The above code would run One or both null statements if even one field value if NULL if both are null then if both are Not Null statements then it would run
If you use AND then even if both are null then The "One null statements" will be executed and if one is NULL then One is null or not null then Not Null statements will be executed.
Hope it works!