Answer:
They do not want to appear divided in front of their armies.
Step-by-step explanation:
When Cassius arrives with his army to the camp accuses Brutus of having wronged him. Soon after that Brutus said to Cassius that he considered him as a brother and convince him to keep the argue inside the tent because: "...Before the eyes of both our armies here, Which should perceive nothing but love from us...".
Also before that when Lucillius told Brutus about the displease of Cassius towards him. He asks Brutus to step aside and gave the message in secret so nobody could hear them.
I hope this answer helps you.