Multiple ways to tackle this one:
If you booked through a travel agent (TA), reach out to your TA and he/she/they can get that taken care of for you. Generally it is assumed that the beds are together, but they can submit the request to separate.
If memory serves, the option (together vs separate) is part of the booking process...i.e., checking a box... so if it was missed, you can try calling Royal and they can make the update.
If you are in a suite (GS or above), the Concierge will be reaching out to you a few days prior to sailing with a variety of information and offers to assist. He/she can help with this request.
As a last resort, you can always catch your cabin steward on embarkation day. They are almost always bustling around to introduce themselves to their guests as they arrive and offering to assist with whatever is needed. Though not ideal since they are usually busy getting cabins in order for other guests too, it is possible.