שולחן ערוך, אורח חיים קמ״א:ב׳
Two people may not read together, rather the one got called up should read and the prayer leader should be quiet, or the prayer leader should read and the one who got called up should not read it out loud. Nevertheless, he must read with the prayer leader so that his blessing should not have been in vain, but rather he should read along quiet enough so that he cannot hear it in his own ears. Rem"a: And even if he can hear himself, this is not a concern, for it is no different than [hearing himself while] praying [the Amidah] as was seen above in siman 101. [His own opinion].
משנה ברורה קמ״א:ח׳
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מגילה כ״א א:י״ח
We learned in the mishna: If one person reads the Megilla or two people read it together, they have fulfilled their obligation.
מגילה כ״א ב:א׳
It was taught: This is not the case with regard to reading the Torah, which may be read only by a single person. The Sages taught (Tosefta, Megilla 3:20): When reading from the Torah, one person reads and one may translate the reading into Aramaic for the congregation, provided that there are not one person reading and two people translating, because two voices cannot be heard simultaneously. And when reading from the Prophets, one person reads and two may translate, as there is less of a need to ensure that everyone hears the precise translation, as the Prophets do not teach halakha. This is the case provided that there are not two people reading and two translating. And when reciting hallel and reading the Megilla, even ten people may read and ten may translate.
תוספות על מגילה כ״א ב:א׳:א׳
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