On June 29, A.D. 67 St. Peter was crucified, head downward, by order of Nero, on a hill overlooking the Vatican. On the same day St. Paul was beheaded outside one of the gates of Rome. The Romans celebrated this feast with splendor equal to, if not surpassing, that of Easter.
There are two stations, one at St. Peter’s and the other at St. Paul’s, but at both Masses the Collects are common to the two apostles indissolubly united in a single prayer.
It is just because this ancient feeling which is common not only to the Romans, but to all the Fathers, that the two Founders of the Roman Church can never be divided. Tradition has preserved the memory of the places in which St. Peter and St. Paul dwelt. It is certain the chair of St. Peter and the chains of St. Paul were carefully guarded. The Vatican Basilica is erected over the burial place of St. Peter; the Ostian Basilica, St. Paul’s – Outside-the-Walls, honors the Apostle of the Gentiles.
(Source: The New Roman Missal by Father Lasance)