Is ねえ、ねえ used by males, females or both?
It is an interjection used to presume upon another's love or to flatter a partner to attract his/her interest.
It is often used in the scene like: when the child behaves like a spoilt child who is such as coaxing his/her parent into buying a toy or something like that for him/her; and when a female lover plays the coquette.
So, it is mostly used by females and also used by children under around ten years old.
Used by both sexes though it may sound feminine.
It can be said that women use it more often than men, but there's no clear-cut gender nuance in this word, unlike how it may seem in fictional works. I guess a "macho man" wouldn't use it at all, though.
In order to understand ねえねえ we must first know the usage of ねえ and なあ. They're both the friendliest interjections to draw the hearer's attention, but なあ carries a feeling of "throwing a word towards the hearer", while ねえ more like "pulling the hearer to this topic". In this sense, it's sometimes described that なあ is "masculine" and ねえ "feminine". This isn't totally wrong, because there is social discouragement against women using なあ (as Standard Japanese) in public. (But young girls do use it a lot within their circle. I know.)
Coming back to ねえねえ, it's reduplicated form of ねえ. So if ねえ is a "nudge", ねえねえ is "nudge-nudge". Imaginably women would use it more casually when men would make do with just once. However, there are also substantial use cases when someone has to tell "you should really listen to me" in moderate manner, regardless of gender.
Hey... the teacher's standing behind you.