The side story quests missable?

My partner and I are both playing the game, trying to stay relatively caught up with each other. Not knowing areas would be locked off with story progression, they started act 2 without finishing off all of the act 1 side stories. Will those missed quests be available to them later, like when the second round of side story quests becomes available normally, they can still do the first set, or have they been missed completely?