One part pays monthly and one pays at maturity over the same term. Can I add up their returns?

You can add the two parts' returns when they have exactly the same term and each part is one specific product whose return was computed at the amount it holds. But that total is a figure over the whole term, not a figure you receive at the end. The monthly part's return reaches you as you go throughout the term, and the other part's return comes once at the end, so next to the total you need to see when each part arrives. Note that the part paying at the end does not necessarily earn more, and neither does the monthly one: the quoted rate is annual whatever the payout schedule, the rate can differ by schedule because the schedule is part of how the product is priced, and the real difference is the timing of the money. A monthly return also does not grow by itself unless you reinvest it, and that is not in any calculation. If you have not chosen a specific product for each part, any 'total' is a guess. Ask the bank for the return of each product at your amount, term and payout schedule, and check the key-facts statement for the payout schedule and the date from which the return is calculated.