Ni l'un(e) ni l'autre ne... means neither one nor the other or neither (of them).
Look at these examples:
J'ai appelé Paul et Hadrien, mais ni l'un ni l'autre n'a répondu.I called Paul and Hadrien, but neither one nor the other answered
L'un/l'une agrees in gender with its antecedent.
Note also that the verb following is conjugated as with il/elle.
Ni l'un/e ni l'autre can also be used on its own to express neither:
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Examples and resources
Qu'en est-il de Julien et Sophie? - Ni l'un ni l'autre ne peut venir.What about Julien and Sophie? - Neither one nor the other can come
J'ai appelé Paul et Hadrien, mais ni l'un ni l'autre n'a répondu.I called Paul and Hadrien, but neither one nor the other answered
Quel parfum tu veux ? Fraise ou vanille? - Ni l'un ni l'autre.What flavour do you want ? Strawberry or vanilla? - Neither.