Of course you are not going to get stereo from one Bose, regardless of the hookup. I would test the various ways and use my ears to see if one sounds better than the other.
The few times I did it, I ran from left and right outs from the keyboard into the stereo inputs on the Bose Compact. Korgs sounded fine, Yamahas not so fine. Yamaha arrangers NEED stereo processing to reach their potential.