Hi,
Lookup function returns only one value from the destination dataset. So, I wonder want you mean by "bring both amount".
Let's assume, in the tablix, you have Dataset1 in it. You could just drag and drop the amount value in the table , then use lookup to have the corespondent Dataset2 value in another column. So that both value would appear in the table.
The expression should be :
=lookup(Fields!ID.value, Fields!dataset2ID.Value, Fields!a_org.Value, "DataSet2")
=Lookup(source_expression, destination_expression, result_expression, dataset)
Check the Lookup definition , see what is the logic for your case should be: Functions - Lookup Function
Regards,
Joy