I myself might be too deeply stuck into logic, but to me they are and will ever be two different characters. As many people already mentioned, it's too obvious and seeming to be forced that the makers in the late phase of the series let their own logic down and thought, "Hmm, we have to create a new phenomenon no one expects to deal with in the following seasons. Any ideas?" - "Hey, why not make Magnum believe Higgins is Robin Masters?" - "Not bad, but this is against the series's logic, consider we originally created both as two." - "Logic? It's not more logical that all American and Soviet agents ever alive seem to be resident in the same street, and that every other inhabitant of Honolulu seems to be British." - "Good point. Who needs internal logic, let's do it."
Another point I haven't found anyone mentioning. If Higgins is Robin Masters, how does he manage to at the same time not only write two different books in two completely different styles but also always to switch between two variants of English language? If he wants to fool the whole world he has to be careful always to write, for example, "favorite" when being RM and "favourite" when being JQH, as well as taking care of certain grammatical and idiomatic differences between UK and US variant - and not to mix it up at even one point. Even if he is able to switch codes whenever necessary and knows both perfectly (at least those differences in written language, of course, aren't that extraordinarily big, but they exist), I imagine it very hard to concentrate on that, especially if not even the editor is allowed to notice (and I personally wouldn't inform him about me having a secret identity ...).
(At least I don't recall anything said in the series about Masters having another nationality than American ... if there is, forget what I said
