One way you should be able to do this would be to change your calculation to be:
if primary.username () = ATTR(secondary.local_identifier) then 1 else 0 End
Basically this will create a true/false statement, where 1 is equal to true and 0 is equal to false. You should then be able to place the calculation on the filter shelf, then just filter to 1.
Hope this helps!
Thank you, Tracy! I will try this shortly. ~ Jennifer