Unless you're travelling on budget airlines (which I assume you're not, since you're flying intercontinentally), it's almost always cheaper to buy any kind of return ticket, including open-jaw returns, than buy the equivalent sequence of single tickets.
Also, a significant advantage of return tickets is that the airline is contractually obliged to get you home if you have a return ticket. If you have two single tickets, they can take you to South America (fulfilling the contract of the first ticket) and then say, "Oops, we're terribly sorry but your flight home is cancelled. Here's a full refund." This fulfils the contract for the second ticket but leaves you stuck on the wrong continent.