I've used this rate many times at properties worldwide. So far I haven't found a property that said no. It is very property specific as to which rates qualify- some places have allowed me to book the first room at a discounted rate with the second at half of a that, while others have said that the first room had to be at full rack rate and then the second would be half that. One UK property recently told me that the family plan was only available on rooms booked with a breakfast package. I usually call/email around to find out what different properties are willing to offer, and then take the best deal.