Learn Finnish: Kummallakin koiralla on jo siru ja rokotus. - Both dogs already have a microchip and a vaccination.

When declining kumpikin, you add the case endings to the first part (kumpi-) and always keep the -kin particle at the very end. The stem for kumpi is kumpa-. Adding the adessive ending -lla triggers consonant gradation (mp changes to mm), giving us kummalla. Finally, stick -kin back on the end to get kummallakin.

Since there are two dogs, why is the verb on in the singular instead of the plural?

While we translate it as "both dogs" in English, kumpikin literally means "each of the two". Because "each" is grammatically singular in Finnish, both the noun it modifies (koiralla) and the verb (on) must stay in the singular form.

Could I use molemmat (both) instead of kumpikin here?

Yes, but it would change the grammar. Molemmat is inherently plural. If you used it, the noun would also have to be plural: molemmilla koirilla on (Both dogs have). Using kumpikin instead of molemmat places slightly more emphasis on the dogs as separate individuals.

How is the word rokotus (vaccination) formed?