Learn Finnish: Minä en antanut numeroa kummallekaan. - I didn't give a number to either.

Kumpikaan is made of the pronoun kumpi (which of the two) and the enclitic particle -kaan. When you add a case ending like the allative -lle (to), it must go between the pronoun and the -kaan particle. So we decline kumpi into kummalle, and then stick -kaan onto the very end.

Why does the mp in kumpi change to mm in kummallekaan?

This happens because of consonant gradation. When adding an ending that closes the syllable (like the double consonant in -lle), the strong mp in the stem weakens to mm. That is why kumpi becomes kummalle.

The English translation says "to either", but doesn't kummallekaan literally mean "to neither"?

Yes, it does! Finnish uses a double negative structure here. Negative pronouns like kumpikaan (neither) always require a negative verb in the same sentence. So, en antanut (I didn't give) pairs with kummallekaan (to neither), which we naturally translate into standard English as "I didn't give... to either."

Just as a refresher, why is the word numeroa in the partitive case here?