Late to this party, but reading the 1935 excavation report (Callendar and Scott) on the Knowe of Yarso states that there is evidence for a second level at about three feet, three inches off the floor of the rearmost chamber ("3A"), and that this level was probably destroyed by stone robbers of later eras. So perhaps split level cairns were more common, they just didn't survive.
I wonder what other cairns show signs of ledges suitable for a shelf and thereby a second floor?
Something is not right. This message is just to keep things from messing up down the road