There are several pairs of standing stones surrounding the Preselis. The ordnance survey 1:25,000 scale map of the area reveals at least six. To my knowledge pairs of stones are not particularly common, and I have no explanation for the significant cluster around these hills. E-mail me if you have any ideas on that.
In addition to the pair of stones which stand as outliers to the Gors Fawr circle, I have pictures of another pair. They lie a short distance away from Gors Fawr, but have no traces of a circle nearby. In keeping with the many other arthurian myths of the area they are known as the Stones of Arthur's Sons.