Macbeth is told by Lady Macbeth that it is Donalbain, one of Duncan’s sons, who
lies i’ the second chamber
It is possible that Donalbain had an attendant with him, but equally possible that his brother, Malcolm, shared the chamber with him. It is therefore more chilling to Macbeth to overhear them as he leaves Duncan’s chamber -
There's one did laugh in's sleep, and one cried
That they did wake each other: I stood and heard them:
But they did say their prayers and address'd them
Again to sleep.
If it is indeed the king’s sons who are so disturbed, then Macbeth’s success in getting away with the crime is so much more miraculous, and it could be seen by him and his wife that they did require spiritual intervention (from the witches) to succeed.
There is, of course, the effect on Macbeth who cannot pray with them, a condition he attributes to his evil deed, which is fatally dismissed by his wife, and which becomes the prophesy of her own demise
These deeds must not be thought
After these ways; so, it will make us mad.
In this scene, Macbeth is in the process of killing the king. After he does it, he comes down and talks to his wife. This is where we find out about the two people.
The two people are servants who are in a room near the king's. Macbeth believes that he heard them wake up after he killed the king. We do not know why one of them laughed, but the other of them cried out (not cried like with tears) "murder." That cry woke them both up. After that, they said their prayers and went back to sleep.
Presumably, the whole point of this is just to illustrate Macbeth's feeling of guilt. We see this because he tries to pray along with them but cannot say "amen."
The two persons referred to in act 2 scene 2 are servants or 'grooms' stationed close to Macbeths quarters referre to as
Line 53: The sleepy grooms with blood.
According to the plot, they are going to be incriminated with the murder. Lady macbeth will paint ther faces with Duncans blood as well as place the daggers close to them;
Line 56: Give me the daggers...
Line 58: If he do bleed, I'll gild he faces of the groom withal, For itt must seem their guilt.
They laugh and cry to symolize the conflictng emotins in Macbeth all of which are overidden by guilt. and fear, as in
"Sleep no more! Macbeth does murder sleep'- the innocent sleep,...
Here the 'innocence" of sleep is used to emphasize the magnitude of the guilt that Macbeth experiences. They laugh because macbeth has accomplished for his ambition and cry because he has lost forever twithin which he commits the atrocity.