I too have booked two rooms many times, however, they do require the names of the other party, and though it is in my name, they have their own reservation number. I’m wondering if the problem is that Disney can’t (won’t) give you the same discount offer on another reservation since the discount has probably expired now.
If you confirmed with a CM prior that you would be able to add a room, ask to speak to a manager and they may be able to work it out.
Maybe you can add the second room to your reservation w/o any adults in the room and then see if they will let you add guests at a later date? Seems silly, but might be a loop-hole.