The starting claim that the two books "Luke" and "Acts" were originally a single volume is not vindicated from any archaeological source nor by quotes from other ancient Christian writers. The real reason behind claiming they were originally a single work is to try to excuse dating the books after the fall of the temple. the script of Acts ends in abruptly with Paul in Rome, and can be dated as AD62, over two years after Festus became governor of Judea and sent him there.
The dating of the books may be commonly stated to be past AD80, but this assertion is not validated by internal nor external evidence.
Irenaeus Heresies III.14,1 mentions Luke with Paul as two working together and verifying each-other, the book of Acts is not shown as combined with the gospel, and again later Irenaeus Heresies III.14,3 separately listed Luke's gospel contents which are unique to him.
Luke's gospel borrowed heavily from Mark, but also from other writings. He collates some of Jesus' parables together in chapters 14, 15 and 16 which are not found in Mark and come from other sources as Luke indicated he had used a variety of historical sources in his opening paragraph to Theophilus.
Acts has two parts, the first centres on Peter and his early ministry (chapters 1-12) and covers until the death of king Herod Agrippa I in AD44, which is obviously researched by Luke. The second half is on Paul's ministry (ch 13-2 where Luke companies him after Troas (ch 16v10) and sometimes appears to have gone absent from Paul until returning back to be with him on his final journey to the temple and subsequent arrest, where he stayed with him until AD62. Much of what he wrote was as an eye witness.
The dating of Acts must be AD62 or AD63 at the very, very latest, and the book of Luke probably AD 59 or 60 as Luke was absent from Paul for two years when he was imprisoned (Acts 24v27).
The two books have always been separate and there is no evidence to suggest otherwise.