If more than one Stonehorn Dignitary enters the battlefield and their abilities target the same opponent, that opponent will skip that many combat phases (over multiple turns if necessary). Skipping one combat phase won't satisfy all of the effects.

Once a combat phase starts, it can't be skipped. A Stonehorn Dignitary entering the battlefield during combat won't affect that combat, even if it's the turn of the opponent chosen as the target of the triggered ability.