BEX filter based on characteristic combination

Hi,

I have 0SOLD_TO and 0CUSTOMER where 0SOLD_TO ( as you know) is referenced from 0CUSTOMER. Let say the user does not select neither customer nor sold to in the report global filter.And let say there are 3 rows in cube.

sold to - customer - key figure

A A 5

A B 10

C C 20

I want to display row 1 and row 3 where sold to and customer are the same.(both are A and both are C. ) .

I think a variable on sold to with type replacement path and referenced from customer will not work.Do I have to write a user exit? If so , can you give clues about the exit?