There were two veils in the Second Temple, aka Herod's Temple. One was from the outside court or court of the gentiles that veiled the Holy Place. The second was between the Holy Place and the Holy of Holies. Only the High Priest could enter the Holy of Holies, but into the Holy Place the other priests could enter. For example, the Holy Place is where the father of John the Baptist had his vision while ministering.
There could have been a lag time between when the inner veil ripped from top to bottom to when the news spread, but there may be another explanation.
In these two pericopes, the timing is different.
In the first, the veil (outer) tears and then Jesus dies.
And the sun was darkened, and the veil of the temple was rent in the midst. And when Jesus had cried with a loud voice, he said, Father, into thy hands I commend my spirit: and having said thus, he gave up the ghost.
In the second, Jesus dies and then the veil (inner) tears.
And Jesus cried with a loud voice, and gave up the ghost. And the veil of the temple was rent in twain from the top to the bottom.
Veil in Luke
Veil in Mark
So, it appears the bible is telling us a time line of the outer veil tearing first, Jesus dying, and the inner veil tearing second.
Who and How
With that in mind, who could have watched this and how (from where)? This question leads to the question of where Christ was crucified?
If one uses the traditional view that Christ was crucified north of the city of Jerusalem, there is no line of sight into the temple. This tradition is where the Church of the Holy Sepelchre is located.
But other research is indicating the crucifixion happened in an area east of the city and thus east of the temple. From that vantage, it could be possible that one could see into the temple. Here are links to the second view.