While panning left and right will work, you won't be able to change the mix with an external mixer located somewhere else in the room. With the XS900, you can assign the LEFT and RIGHT voices (or, actually, any combination of audio) to Sub 1/2. These can be completely separate channels to the mixing board while the styles, MIDI, Song, etc. (mixed in the keyboard) are delivered to two other channels in the mixer. There are several other things the SX900 can do that the OP might find useful. The vocal effects in the SX900, especially, might come in handy on stage, too.
Jim