Assuming that you have a total on each of two subforms and you want to add these together and show result on the main form.
You the following expression in the text box on the main form. You will need to replace some of the expression with the real names used in your database:
=[NameOfSubForm1].[Form]![NameOfFirstTotalTextbox] + [NameOfSubForm2].[Form]![NameOfSecondTotalTextbox]