Two Revel m20s as center: good idea?

Two are almost always WORSE than one. The interaction of the radiation from two sources of the same signals results in an irregular dispersion pattern. That said, some have made a case for one above and one below but, ideally, only one is needed.

Kal

The problem is less of an issue with stereo since there are distance-related phase shifts from a real central voice/instrument when recorded in stereo. If mono-miked and split, then the problem occurs and is one of the causes of issues with vastly multimiked recordings. As for the problems with multidriver systems in the crossover regions, that is certainly relevant and is the reason why the vast majority of speaker systems are vertical arrays. This confines the the interference to the vertical plane. Thus, a vertical pair of centers is less pernicious than a horizontal pair.

Kal