Hi Perlin. You would say, "I haven't seen either of them yet."

I have not seen either of them.

This is a difficult one to understand. " I haven't seen either of them" = is correct it means you have not seen (both of only two possibilities) If you own two dogs and they are both out roaming and haven't yet come home for their feed as usual. you would say this. "I haven't seen either of them" You could not say it for anything / about anything above two (a couple). Neither technically means not+either and is used after a negative statement. So to correct the first sentence you should swap it around like this and remove the "have" and replace with "haven't" "I haven't (- negative) seen neither of them" (one) can be omitted from either sentence because you cannot see (have seen) "one of them" and also not seen either of them in the sentence "I have seen (you saw) neither (one) of them" = ? logically. "I have not seen neither of them" = you have not seen any of a group of two. This means you can use neither or either so long as you remember to use neither after a negative statement. Or use neither after a statement about only two possibilities "neither John nor Mary are coming home tonight" And for all other instance of either / neither /not /nor / see the link https://www.lawlessenglish.com/english-mistakes/either-and-neither/ It is the logic of how computers work and can get a bit confusing. For simplicity stick with either and when using neither put the negative statement first. If ever you get into difficulty simplify by using "any" "any of them" "any one of them" . Or just learn it as best as you can then after any exams if that is what you are planning. Revert to saying "I haven't seen any of them" or "I haven't seen any one of them" "I haven't seen either of them will always be the best choice". Because it will avoid any doubts, debates and discussions about whether you or I have used a double negative or not, or whether you or I are using the active or passive voice. Or whether double negatives and active and passive are being mixed up.

I have not seen either of them.

It's more correct to say "I have seen neither one of them." You can also say "I have not seen either of them." Saying "haven't" and "neither" is like having two negatives and while you can be understood it does not sound natural