Learn Finnish: Ostin kahdet kiikarit, jotta voimme kumpikin katsella. - I bought two pairs of binoculars so that we can both watch.

Because kiikarit (binoculars) is a plural word, just like in English. When you count plural nouns, matched pairs, or things with two symmetrical parts, you must use the plural form of the number. Here, the number kaksi becomes the plural nominative kahdet. If you used kaksi kiikaria, it would sound like you bought two separate lenses of a binocular.

Since the binoculars are the object of ostin (I bought), why aren't they in the genitive/accusative case?

While singular total objects take the genitive-like accusative case (e.g., ostin auton), plural total objects are always in the nominative plural (e.g., ostin autot). Because kiikarit is inherently plural, it takes the nominative plural form here, and the plural numeral kahdet agrees with it.

Why does the sentence use kumpikin instead of molemmat for "both"?

While both can translate to "both" in English, kumpikin emphasizes the individuals acting separately ("each of us two"). Because the speaker bought two pairs of binoculars so that each person can hold their own pair and look independently, kumpikin is the most natural choice. Molemmat usually implies acting together as a pair.

Why use the verb katsella here rather than the basic verb katsoa?