Learn Croatian: Ravnopravan udio pripada obama suautorima. - The equal share belongs to both co-authors.

The verb pripadati (to belong to) requires the Dative case for whoever is receiving the item. While you are probably used to oba acting alongside the genitive-like paucal form in the Nominative or Accusative, here it must decline fully into the Dative case. Because suautor is a masculine noun, the Dative form of "both" is obama.

If the co-authors were women (suautorice), how would obama change?

It would change to objema. When dealing with feminine nouns in the Dative, Locative, or Instrumental cases, the form is objema (so the phrase would be objema suautoricama). For masculine and neuter nouns, it is always obama.

Is udio (share) a neuter noun because it ends in "-o"?

No, udio is actually masculine, which is why the adjective ravnopravan has no special ending. The noun originally ended in an "-l" that transformed into an "-o" over time. Because of this historical shift, the stem changes when it declines—for example, the Genitive form is udjela.

Could I use the word koautor instead of suautor?