|Palatine Chapel in Palermo
The answer is actually missing in the texts of yesterday's Offices and Mass. On the feast of Ss. Peter & Paul there is hardly any mention made of St. Paul in the Mattins readings (other than allusion by St Leo), the antiphons on the Major Hours, and none at Mass save for the collect. This underscores a subtle oddity in the liturgical history of the Eternal City: June 29th was not the single feast of Ss. Peter and Paul, but it was the feast of both Saint Peter and Saint Paul, two feasts in one day.
As Pierre Batiffol recounts in his History of the Roman Breviary, feasts in the first few post-Nicene centuries were often unique to the churches of Rome and celebrated in honor of the saints whose relics were contained there. By the time of St. Gregory the Great Rome had declined to a meager population of about 30,000 from its Imperial peak of a million. Yet the devout in his small population could ambulate through the vast city to churches that held the relics of saints and celebrate the vigil and Mass with the Lord Pope of the City with relative ease, the origin of the "stational churches" of great feasts, vigils, and fasting days that are still observed during Lent to this day. As such, feasts of multiple saints usually pertained to the saints buried in a given church. Sancti Apostoli in Rome holds the bones of Ss. Philip and James, and so the transfer of their relics to that church elicited their feast on May
Instead, the Basilica of Saint Paul Outside the Wall and the Vatican Basilica of Saint Peter would each have observed the feasts of their patronal saint whose relics rested beneath the main altar. In times of old the Pope would begin the evening at the Basilica of Saint Paul Outside for the vigil (Vespers, Mattins, and Lauds) and continue until day break, when he would travel to Saint Peter's on the Vatican Hill for the celebration of the Mass. Because the Mass took place at the Vatican Basilica the Missal texts that come down to us are for the patronal feast of that particular church just as any pre-John XXIII hand missal gives St. Peter's as the station for June 29. The 10th century Gregorian Sacramentary gives June 29th as the feast of St. Peter, despite mentioning both Apostles in the oration, and June 30th as the feast of St. Paul. By the Middle Ages June 29th was conceded as the feast of both Saints, in accordance with ancient prerogative, and the uniquely Pauline Offices and Mass of June 30th were rebranded as the Commemoration of Saint Paul giving the Doctor Gentium the due he would one have received on June 29th in the Basilica that assumes his name.