This is what I see. It says nothing about double OBC for 1. It explicitly says single.
Up to $400 Onboard Credit per stateroom (up to $200 per guest) is applicable to the first two guests in a stateroom and amount awarded varies based on cruise length. Onboard Credit is awarded as a credit (“Credit”) that is applied to the guest’s folio and is awarded in the currency corresponding to the ship’s onboard currency. Voyages 1-3 days in length will receive $25 Credit per guest. Voyages 4-6 days in length will receive $50 Credit per guest. Voyages 7-11 days in length will receive $100 Credit per guest. Voyages 12 days or longer will receive $200 Credit per guest. Offer does not apply to additional guests than guests indicated above. Guests with single occupancy will receive single, per guest Credit amount ($25-$200). Credit must be used on the single voyage that it was awarded in connection with, is not redeemable for cash, cannot be used in the casino, is not applicable during the land portion of cruisetours, and expires at the end of that voyage.