Two report parameter with or condition /either should be selected

hello,

In my ssrs report builder 3 i am creating report for sharepoint list,where i have placed 2 report parameters ,when i select only one parameter it gives error as please give value for parameter,but if checked allow blank or allow null ,i am getting all values as report parameter is null or blank as the value of the report parameter is null or blank.

so i need to select only one parameter and if i didnt give any value in another parameter it should take all values from the list.

please help.

thanks