I recommend that you use some good Bible software, cut and paste Matt 28 and Luke 21 into a 2 column table in Word, and then do a side by side comparison of the two sections. There are too many differences to harmonize to claim they are speaking of the same event. Even the questions are different.
A Composite of the Four Gospels - Check it out! This is a side by side comparison of the Gospels. I use it quite often in my studies.