You can simply book them yourself.
DCL CMs check to see who is sailing B2B themselves and they inform you of the turnaround procedure without you having to notify them.
In some circumstances, when you search for a room online, it will not show up in inventory because there are too many rooms available in the category you picked. If you picked a room for the first leg, you want it for both cruises and it's not showing up for the second leg, that would be a reason to call DCL. Of course, if it is already booked, they will not change the existing booking to accommodate you.