Unbelievers make heavy weather of the fact that Mark mentions one angel at the tomb while Luke mentions two. But if you ask me, I’d chalk this up to Lukan/dominical numerology. Consider the stereotypical use of “2” in parabolic discourse:
“A certain moneylender had two debtors. One owed five hundred denarii, and the other fifty” (Lk 7:41).
“There was a man who had two sons” (Lk 15:11).
“I tell you, in that night there will be two in one bed. One will be taken and the other left” (Lk 17:34).
“There will be two women grinding together. One will be taken and the other left” (Lk 17:35).
“Two men went up into the temple to pray, one a Pharisee and the other a tax collector” (Lk 18:10).
Numbering things by two seems to be a narrative cliché.
Consider some other parallels:
While they were perplexed about this, behold, two men stood by them in dazzling apparel” (Lk 24:4).
“And behold, two men were talking with him, Moses and Elijah” (Lk 9:30).
“That very day two of them were going to a village named Emmaus, about seven miles from Jerusalem” (Lk 24:13).
So Luke is fond of grouping things by two. Of course, this doesn’t mean that every use of “two” must be conventional. But the use of “two” as a storytelling convention ought to forewarn us not to press Lukan usage with mechanical literality.