Compare two columns from different datasets

Hi, I have two datasets from MDX queries to two different cubes in a report, each dataset is linked to a table in the report layout.

One table is from a real sales' cube and the other one is from budget's cube. Anyway I want to create another table with a column that compares (subtract) one column from one table and another column from another table. Is this possible

I tried with Reportitems("textboxinfirsttable").Value, but it returns always the last value in the table.

I don’t know what to do, pleas HEEELP