A priest mentioned this briefly in his Homily yesterday. I will most likely do it injustice, but here I go:
They are celebrated together (1) partly because their combined feast is a Holy Day of Obligation (though not obligatory in many places) and (2) their apostolic lives, accomplishments, and martyrdom together helped found the Church.
The Church would not have gotten off to the start that it did without the extraordinary efforts of these two. St. Peter at his chair, leading the Church from Antioch and eventually Rome and St. Paul traveling around evangelizing and catechizing the nations.
Their lives together remind us of the founding of the earthy Church, after Christ’s Resurrection and Ascension.
I pray this helps.
God Bless!