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Many people reading simultaneously out loud: 
One fulfills his obligation of hearing the reading even if many people are reading the Megillah simultaneously, [as since the Mitzvah of Megillah is beloved by people, one therefore concentrates to overhear a single reader from amongst the many]. Nevertheless this only applies if one in truth is able to concentrate and hear every word, otherwise he does not fulfill his obligation.
Lechatchilah: Initially one is not to read the words out loud together with the Chazan and he is not to help him with the reading. This applies whether one is reading from memory or from a Chumash or even from a Kosher Megillah.
 Michaber 690/2
 Michaber ibid; even if ten people are reading it simultaneously. [ibid] This applies even initially. [Erech Hashulchan 690/1; Shaar Hatziyon 690/4; Kaf Hachaim 690/8]
 Taz 690/2; M”A 690/3; This is unlike the law regarding the Torah reading in which case two people cannot read it simultaneously. [ibid]
 M”B 690/4; Kaf Hachaim 690/10
 Rama 690/4
 The reason: The reason for this follows the same reasons mentioned in the previous Halacha that perhaps a listener [including the helper himself] will listen in to the words of the helper who is reading out loud rather than the Chazan who is reading from a Kosher Megillah. [M”B 690/14; Kaf Hachaim 690/28]
 Rama ibid
 Michaber ibid
 Shaar Hatziyon 690/11 based on reason of M”A 690/6 that the other listeners may focus on the words of this person, and this person reading, which is not the Chazan, does not have in mind to fulfill the obligation of others. [ibid]