Add 2 Totals from different tables

such companions, I have the following problem ...

I have two tables which use different DataSet, each table gives me back a total, and I want to do is add the two totals and put the result in the report header ....

I tried that but it gives error ... "not allowed childish aggregates in the header or footer"

Use the following expression and did not work "

= Sum (Fields! JobBilledExVat.Value," Dataset1 ") + Sum (Fields! JobBilledExVat.Value," Dataset2 ")"

any suggestions ... Greetings!