If both placeholders were purchased on the same cruise, you might be able to book 2 rooms if someone in the 2nd room is eligible for pre-booking on that date.
If your placeholders were purchased on different cruises, then you wouldn't be able to use them on the same cruise because you would need to be the lead guest in each room to use the placeholder. You could ask about changing the name on one of them, but I don't think that's possible after the placeholder is purchased.