Yes, it's still a single ticket and if there are significant changes on one flight you can cancel or rebook entire ticket. In some cases, it may be cheaper to purchase the two fares on two separate tickets (due to fare combination restrictions on certain cheaper fares which may require upfaring to less restrictive higher fares in order to combine on one ticket), but that's fairly rare these days. And as separate tickets, you wouldn't be able to cancel both if there was only a change on one flight.