Huntersquoy (ORK 23) on Eday, and Taversoe Tuick (ORK 49) on Rousay, are both of double-decker construction. Both are in round cairns, both have Bookan-style chambers and both have the entrance passages to the two levels arranged at 180 degrees to each other.
Both levels at Taversoe certainly seem to have been built at the same time, so it's not a multi-phase monument. There was no communication between the two levels at either Huntersquoy or Taversoe, the two chambers were completely separate.
We've just put up a full piece on Taversoe Tuick which includes a high resolution full spherical VR tour of the entire interior if you want to look around in there:-
http://www.megalithics.com/scotland/taversoe/tuickmain.htm