The Bar Mitzvoh Unison

Shulchan Oruch.-O.H. 141:2 Mogen Avrohom forbids more than one person to recite brochos simultaneously outloud, in order to ensure that every word of the brocho is heard clearly.

In one fo the rare rulings, R. Moshe Feinstein also ruled that two Bar Mitzvoh boys cannot read the maftir and haftoroh sections in unison (Igrois Moishe, O.H. I-102).