My setup was always two speakers for these effects also.

First, on my Roland U-20 I used two oscillators for my string patch. I detuned each one by a hair from perfect pitch (one slight above "0" one slightly below 0)and then panned each oscillator to a different speaker. The result as I recall was a wall of sound of strings. They were swirling all around me....very filling.......very inspirational.

Second reason......I was playing an instrument with one hand on the DX7 and one on the U-20. Each patch was carefully panned. I had the speakers just enough apart to "separate" the sound. So if i was playing "trumpet" on one keyboard it would be playing out of say the left speaker and the sound from the other hand (trombone, clarinet, etc), would emanate from the other. I wanted to accomplish an actual band sound for the listener....the different "horn" sounds coming from different areas of the stage. Don't know if it ever worked.....I couldn't walk into the audience zone to hear myself play.