The very reason for using a centre speaker is to avoid a dual-mono, phantom-imaged, comb-filtering centre channel that comes with sending the same exact information to 2 speakers. This is all the more critical because of what usually gets reproduced through the centre speaker: vocals and dialogue. The human voice is a point source (no one speakers in dual-mono) and is more realistically reproduced via a single speaker. Using 2 centre speakers defeats all the advantages of going beyond a 2-speaker front soundstage.