Is there any historical evidence for Caesar Tiberius (commonly referred to Tiberius Caesar) having a co-regency with his predecessor Caesar Augustus? I have read (in a book that I don't remember the title nor author) that some believe there was a co-regency. In an article from Answers In Genesis (see Conclusion), the author writes
[S]ome have postulated a co-regency (joint rule) of Tiberius and Augustus during the last few years of Augustus’s reign. However, there is no reliable ancient historical evidence for such co-regency.
Is there "no reliable ancient historical evidence" for a co-regency of Tiberius and Augustus? Is there any evidence at all?
For a little background, Caesar Augustus was the first Emperor of the Roman Empire. He succeeded the dictator Caesar Julius (commonly referred to as Julius Caesar) and was succeeded by Caesar Tiberius. Some claim Tiberius was co-regent with Augustus for a couple years prior to Augustus' death. A co-regency is the situation where a monarchical position (such as king, queen, emperor or empress), normally held by only a single person, is held by two or more.
The article mentioned is about when Jesus Christ died. I shall explain why a co-regency of Tiberius and Augustus is important to Christian apologetics. Had there been a co-regency, Jesus could have started His ministry as early as AD 26 or 27, allowing for AD 30 to be the date of Jesus' death. Had there been no co-regency, His ministry would have started at about AD 29, putting His death around AD 33.