Yep - Definitely doable, but only where there is inventory of more than one room type on the night(s) you need. Perhaps that is the issue?
I just used a PC browser to do a check at the Sofitel Sydney DH, and you select first the number of rooms, and adults per room, and then at the view with the room types, you have the choice of selecting from a drop-down how many rooms of a given room type and rate you want. You could select - for example - a king flexible, and a twin promo rate, both on the same. They are added to the booking on the right of the screen that you then proceed with.
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Cheers,
Matt.