Why does Brutus suggest that he and Cassius talk inside the tent

I think you have the wrong scene - there are only a few lines in this scene and Cassius and Brutus are not both in the scene. In the scene before, however, these two talk about the uncertainties of the future and of the possibility they may not see each other again. They discuss some omens that have shown that the battle may not turn out their way and that this meeting may be "forever and forever farewell."