Whilst the recent archaeology describes a pair of stones, the OS map of 1843 calls it ‘Circle’, with later editions saying ‘Standing Stone’ and ‘Circle’.
Whilst this initially suggests a stone circle to the enthusiastic and optimistic megalithomaniac, I think it actually refers to the adjacent tumulus. They are so close as to appear to be a single monument, especially to earlier cartographers with their lesser knowledge of prehistoric chronology. The same area denoted as ‘Standing Stone’ and ‘Circle’ is now ‘Standing Stone’ and ‘Tumulus’; it must just be that the stone was omitted from the 1843 map.
The site was apparently featured in Archaeologia Cambrensis 1974 (p.13-42), and also the Royal Commission on Ancient Monuments (p.378 of Pembroke), but I’ve not seen either of these documents so I’ve no idea if they definitively clear the matter up.