You're still going to need an amplifier, a two channel (two output) amplifier. Run the Left output to one channel, and the Right to the other. Place. If you have separate speaker enclosures place them about six feet apart, or the stereo image will be too pronounced.
... some keyboard/guitar amplifier manufacturers provide combo amps with stereo speaker arrays pointing slightly off axis from each other, or a "stereo link" output to plug in a separate speaker for one of the channels.
Edit: Jack brings up a good point. My answer is predicated on the use of instrument amplifiers and speakers, not home stereo equipment which would not be up to the task at any appreciable volume.