You always have to pay for double occupancy for cruises no matter how you book it. II cruise exchange is really just a cruise booking portal and not a true exchange. This is why you cannot do a cruise exchange if you have already deposited your week. In a nutshell if you attempt a cruise exchange and during the process you change the default two adults to one, the price screen is going to be double.
One big problem with the cruise exchange is that the price can be even higher than booking elsewhere and not giving up your week for the exact same cruise. Not always but enough to make it silly.