67 Let us now proceed, therefore, to look into the rest of this passage, and see how in other respects these statements are quite consistent with what is given by Matthew. For Luke tells us, with the utmost plainness, that two angels were seen by those women who came to the sepulchre. One of these angels we have understood to be referred to by each of the first two evangelists; that is to say, one of them is noticed by Matthew, namely, the one who was sitting outside upon the stone, and a second by Mark, namely, the one who was sitting within the sepulchre on the right side.
But Luke's version of the scene is to the following effect: “And that day was the preparation, and the Sabbath drew on. And the women which had come with Him from Galilee beheld the sepulchre, and how His body was laid. And they returned, and prepared spices and ointments; and rested the Sabbath day, according to the commandment. Now upon the first day of the week, very early in the morning, they came unto the sepulchre, bringing the spices which they had prepared. And they found the stone rolled away from the sepulchre.
And they entered in, and found not the body of the Lord Jesus. And it came to pass, as they were much perplexed thereabout, behold, two men stood by them in shining garments; and as they were afraid, and bowed down their faces to the earth, they said unto them, Why do you seek the living among the dead? He is not here, but is risen: remember how He spoke unto you when He was yet in Galilee, saying, The Son of man must be delivered into the hands of sinful men, and be crucified, and the third day rise again.
And they remembered His words. And they returned from the sepulchre, and told all these things unto the eleven, and to all the rest.” The question, therefore, is this, how can these angels have been seen sitting each one separately—namely, one outside upon the stone, according to Matthew, and another within upon the right side, according to Mark,— if Luke's report of the same bears that the two stood beside those women, although the words ascribed to them are similar? Well, it is still possible for us to suppose that one angel was seen by the women in the position assigned by Matthew, and in the circumstances indicated by Mark, as we have already explained.
In this way, we may understand the said women to have entered into the sepulchre, that is to say, into a certain space which had been fenced off within a kind of enclosure, in such a manner that an entrance might be said to be made when they came in front of the rocky place in which the sepulchre was constructed; and there we may take them to have beheld the angel sitting upon the stone which had been rolled away from the tomb, as Matthew tells us, or in other words, the angel sitting on the right side, as Mark expresses it. And then we may further surmise that the said women, after they had gone within, and when they were looking at the place where the body of the Lord lay, saw other two angels standing, as Luke informs us, by whom they were addressed in similar terms, with a view to animate their minds and edify their faith.
Source: The Harmony of the Gospels (New Advent)