Luke: But when you see Jerusalem compassed with armies, then know that her desolation is at hand. Then let them that are in Judea flee unto the mountains and Jerusalem shall be trodden down of the Gentiles (Luke 21:2021, 24).
These are two separate occasions. Luke's account took place at the fall of Jerusalem in 70 AD -- followed then by the Diaspora and times of the Gentiles just as Luke recorded that it would.
Matthew's account is yet future, occurring after the times of the Gentiles and the Diaspora.
The Josephus stuff was interesting though.
Hmmm...I disagree with that type of thinking for it's too unbelievable to think that Jesus' words meant two different time periods. Matthew wrote to the Jews, and Luke wrote to the Gentiles who weren't all that acquainted with the prophecies of the OT. In 70 AD there were many gentile Christians intermingled in the population of Jerusalem, and Lukes plain words gave them the warning of the prophecy. Notice that Luke didn't have to give a warning about the Sabbath day - Gentiles ignored particular days.