Two nuclei combine to form one nucleus in nuclear fission.

False

Explanation:

In a nuclear fission, big nucleus of a radioactive atom splits into two or more smaller atoms.  

In a nuclear fusion two nuclei combine to form a single nucleus.

Fission and Fusion are two opposite process.

Fusion is an extremely rare process as there is no methodology developed so far which can stand against the repulsion force of two nuclei. However nuclear fission is now used in generation of electricity.  

Thus, it is clear that Fission is division of nucleus while fusion is combination of two or more nuclei.