These keyboards all sound better in stereo. But then you get into a separate discussion about whether stereo is truly possible in a large room or hall especially filled with moving people, and where the positioning of the speakers may not be optimal, because true stereo only happens in a very small area where the sound channels are properly separated. But then there is another set of opinions that this area may be much greater when using modern column speakers like Maui or Bose.
So who knows? You almost have to find out for yourself by playing recorded music from the keyboard while walking round the room. However, having said all that it seems that many people think it does sound better with two. I have personally never tried it, I think there is more than a fair chance that it only sounds better to the player, because they happen to be in the approximate right position.
Mike