Call into Royal and specify which sailing you want it applied to. Like a future cruise credit they are name specific. Fortunately funds under one name can be applied towards purchase for a different name.
You get to chose which cruise it is applied to. It's blanket applied so it may be worth it to use one and save the other for a different cruise (assuming you had two people booked with the same or similar stuff) and supplement the rest with cash if you are planning to do a second cruise as you can not save anything for another cruise. Once the credit is applied it has to be used on that cruise but anything not used ahead of time should be applied to credit you can use onboard for gratuities and purchases.
