The statement is true. The plan was for Macbeth to murder Duncan with both of the two grooms' daggers and then smear their faces with the king's blood so that they will look guilty. Lady Macbeth has already drugged them so that they will put up no resistance. The Macbeths want to create the impression that someone has bribed the grooms to commit the murder. They plan to use both grooms' daggers so that both will look guilty. When Lady Macbeth is waiting for her husband to return from Duncan's chamber in Act II, Scene 2, she hears him cry offstage:
Who's there? What, ho?
She says to herself:
Alack, I am afraid they have awaked,
And 'tis not done. Th' attempt, and not the deed,
Confounds us. Hark! I laid their daggers ready--
He could not miss 'em. Had he not resembled
My father as he slept, I had done't.
Here the stage directions call for Macbeth to enter with two bloody daggers. After some incidental dialogue, she asks:
Why did you bring these daggers from the place?
They must lie there: go carry them and smear
The sleepy grooms with blood.
Macbeth seems to be in a trance. He is horrified with what he has just done. He also seems frightened. He thought he heard a voice crying out "Sleep no more!" to all the house. He doesn't explain why he returned with both the bloody daggers. Evidently in his abnormal psychological state he simply forgot that he was supposed to leave them. He tells his wife:
I'll go no more.
I am afraid to think what I have done;
Look on't again I dare not.
So Lady Macbeth is forced to take the daggers back to Duncan's chamber and smear the grooms' faces with Duncan's blood.
Shakespeare's real purpose in having Macbeth return with the two daggers is to make a spectacle of as much blood as possible. Since Shakespeare chose not to try to stage the actual murder, he did the next best thing by having Macbeth return covered with blood and carrying two bloody daggers to prove that he had killed Duncan. Then when his wife takes the daggers back to Duncan's chamber and smears blood on both grooms, she returns with bloody hands herself. Both husband and wife make a big display of the blood for the sake of producing an emotional effect on the audience.
Macbeth holds up his hands and says:
What hands are here? Ha! They pluck out mine eyes.
His wife shows her own hands and says:
My hands are of your color, but I shame
To wear a heart so white.
By now the knocking has begun, which is the subject of Thomas De Quincey's famous essay, "On the Knocking at the Gate in Macbeth.". It will continue until the drunken Porter finally opens the gate in Act II, Scene 3. The purpose of the prolonged knocking is to force Macbeth to go down and see why nobody is opening the gate. This will then force him to be present outside Duncan's chamber when Macduff discovers the dead body. Both Macbeth and his wife had planned to be in bed pretending to be sound asleep when the body was discovered. But Macbeth, who is already in a state of nervous shock, has to try to look innocent and astonished while Macduff wakes up everyone in the castle. The discovery of Duncan's both is far more effective with the guilty Macbeth present than it would have been otherwise.