Many keyboards require stereo (therefore two speakers)to properly reproduce some voices, particularly piano. With one speaker you do not get the proper sound output. Conclusion, you should have two speakers. A converse opinion has been expressed that if you are playing, other then in a small room to yourself, then the proportion of your audience that can listen within the "stereo sweet spot" is small, therefore stereo is irrelevant. I recall "Travellin Easy" on another forum I think, expressing this opinion.
Keith
PS I use two Behringer Truth B2031's with my Tyros 2, KN7000 and Motif ES6

[This message has been edited by KeithB (edited 08-24-2007).]