The two Cape Parrot species concept is fairly new, super political, and not widely accepted. The IUCN rejected the proposal and aside from a handful of biased scientists, conservationists, and breeders that can benefit from the split, there is no consensus that they are in fact different species.
The motives of the scientists who support the species split are quite questionable. Not only can they make money by supporting the split, they can also boost their fame and egos. However, the data is hardly convincing at all that Capes cannot interbreed across subspecies. If you look at the largest Congo African Grey and compare it to the smallest Timnehs, you'd be far more convinced that they are different species than any of the most distant Capes. Nonetheless, the unified species of Grey Parrots is not questioned but the South African government along with conservationists and scientists want to benefit from financial grants that endangered species can receive. And since the other two subspecies of Cape Parrots are hardly endangered, they cannot get their money without calling it a separate species first. So if you read the papers supporting speciesation you'll get a strong feeling that they had the end result in mind first and then mangled together every excuse possible to achieve it. Poor science at best.
Therefore, I believe it is safe to say that there is only one species of Cape Parrot,
Poicephalus robustus. The Cape Parrot species is divided into 3 subspecies:
P. robustus robustus - This is the nominate, the one that is bordering on extinction, and the only one that comes from anywhere near the cape in South Africa. These are slightly smaller and have a yellower coloration to their plumage.
P. robustus fuscicollis - This is the brown necked Cape Parrot distinguished by a maroon hue in the grey of the neck. This subspecies comes from much the same region as Senegal Parrots in west Africa.
P. robustus suahelicus - Finally this is the grey headed Cape Parrot with a greyer neck plumage and the largest of the Capes. These come from southern to mid-african countries.
Here is a map I made of their distribution in the wild:
