If Moldgraf Monstrosity’s ability can’t exile it (perhaps because it’s not still in the graveyard when the ability resolves), the two creature cards are still returned to the battlefield.

If two Moldgraf Monstrosities die simultaneously, the first ability to resolve could return the other Moldgraf Monstrosity to the battlefield. If it does, the second Moldgraf Monstrosity’s ability won’t exile it but it will return two more creature cards to the battlefield.