P. subdiphylla was described by English botanist V. S. Summerhayes in 1942 based on a single specimen collected in 1935.

The conical pseudobulbs, each measuring 2–4 cm (0.79–1.57 in) long and 5–10 mm (0.20–0.39 in) wide, arise in clusters from a short, creeping rhizome.

[2] The inflorescence is a simple raceme measuring 9.8–14 cm (3.9–5.5 in) long and bears up to 5 flowers.

[3][4] P. subdiphylla is listed as endangered by the International Union for Conservation of Nature under criteria B1ab(iii) and B2ab(iii), based on the small number of locations at which this species is present and the threat of habitat decline.

P. subdiphylla is present within the Nguru South Forest Reserve, a protected area in the Nguru Mountains, however, other populations occur in unprotected areas that are at risk of deforestation due to agriculture.