Coflein notes this is a “stone pair”, the second stone being the triangular recumbent next to the upright block:
Troed Rhiw stone pair are a monolith, measuring 1.67m high by 1.3m by 0.94m and set in a 3.0m diameter cairn; with a recumbent stone, measuring 1.7m by 0.81m by 0.4m, lying at its foot, all being set on a cultivation terrace.
There are more, possibly placed, blocks along bridleway line running westwards from here along the valley in the direction of Nant Tarw, but these are not listed on Coflein.