Both of them have been mentioned in the chapter.In Act 2 Scene 2, Decius Brutus is seen and later in the chapter there is only Marcus Brutus. Wherever there is a dialouge of Decius Brutus, his full name has been mentioned whereas for Marcus Brutus, only Brutus has been mentioned.Decius Brutus is the one responsible for convincing Caesar to come to the Senate house so that the conspirators can murder him and the one responsible for subverting and manipulating Calpurnia's dream. Decius Brutus is a co-conspirator. Marcus Brutus is one who kills Caesar and asks Mark Antony to join the conspirators and gives speeches and dies in the end. Actually, Marcus Brutus is a part of Caesar's murder but he is fooled by the conspirators. He is unaware of the fact that they are murdering Caesar due to jealousy and not because of ambition.Both of them are Caesar's friends and betray him. Decius Brutus betrays him knowingly whereas Marcus Brutus betrays him unknowingly.The conspirators very cunningly make Marcus Brutus (Caesar's close and trusted friend) a part of their plan to gain the public's trust. Marcus Brutus, unknowingly is used by the conspirators because they need a person whom the Romans trust to fool them. When Marcus Brutus gets to know that Cassius has used him to kill Caesar (because Cassius is jealous of Caesar's power and popularity) , he kills himself.That's how the story ends. I guess I've cleared your doubts. Feel free to ask anymore questions.

MARCUS BRUTUS