Why does there need to be two Adorers for every hour? 

The Church requires two Adorers to ensure that the Blessed Sacrament receives the proper reverence and care that the Blessed Sacrament demands. Because it really is Jesus, the Church wants to make sure that the Blessed Sacrament is never left unattended during Eucharistic Adoration. Having two Adorers serves as a safety in case one Adorer is running late. Having two Adorers also provides a level of security and peace for those who have come to pray, knowing that there is always someone else present.