The Gemara says that one fulfills his obligation of Hallel when he hears it recited by ten people together, Even though there is a principle that "two voices cannot be heard at one time," Hallel is different because it is something which is "beloved to them," and therefore multiple voices can be heard at one time. (Rashi on 27a, DH d'Chavivah, explains with regard to hearing the Megilah recited by two or more people at once that "since it is new to him" it is beloved, meaning that since one does not recite the Megilah often, except once a year, it is beloved to him. Alternatively, it is beloved to a person because of the praise of Hash-m that it includes. The ME'IRI (21a) writes that Hallel is beloved because it contains mention of the miracle of Yetzi'as Mitzrayim.)

(It is worth noting that the Me'iri there says that there is a Girsa which we do not have that does not include the word "uv'Hallel... even if ten people read it." We find also that TOSFOS (DH uv'Megilah) did not have that word in his text. According to that Girsa, one is not Yotzei by hearing Hallel read by two or more people at once.)

It seems that a source can be brought from the Torah that two or more voices saying the same thing at once can be heard from the command given to the Kohanim to bless the people, even when many Kohanim are reciting the blessing together, even though the main point of the blessing is for the people to hear it, as Tosfos writes in Sotah (39b).

Kol tuv,

Mordecai Kornfeld