In 1545, Spanish sailor Ortiz de Retez thought that the island's dark-skinned inhabitants (who called it Papua, among other names) resembled people from Guinea, so he called it Nova Guinea. The name stuck.
A follow up could well be "why are there four Guineas?" PNG and Guinea (or Guinea-Conakry, to be more accurate) are covered but there are, in Africa, two more. Firstly, Guinea-Bissau, which is next door. This is separate as it was a Portugese colony rather than French. Independence was gained after a long war which, confusingly enough, spilled into Guinea itself after they have support to Guinea-Bissau rebels.
This leaves Equatorial Guinea which is, obviously, on the equator. This means it is also nowhere near either Guinea or Guinea-Bissau. This was previously Spanish Guinea located as it is in the Gulf of Guinea which means that the two other African Guineas aren't actually located near anywhere called Guinea. Why are they all Guineas then?
The Portugese are to credit for this for adapting a word to Guinea (there are a couple of stories as to why but I will go with the following one) which referred to Ghana and Guinea referred to a good portion of west Africa including Ghana itself (Dutch/Danish Guinea), Togo (German Guinea) and also Cameroon (also a bit German Guinea).
So, while you mention two in your question, really, there are four Guineas and there could easily have been many more.
Is there any linguistic connection between Guinea and Guyana/French Guiana?
hi! I'm a little late to this, but anyway, there have been a few similar questions in this sub, so do check these out for previous responses
Why did so many countries get named 'Guinea' when they were colonized?
What is the etymology of the word "Guinea", and why is it so widespread?