Hi Bernie,
I ran several tests with one Bose using just one cable into L/R and then the Hosa Y-connector.
On certain styles certain guitar parts and/or certain drum parts did not sound right using one cable. Something was missing or the volume was to low.
Using the Hosa Y-connector does not give me stereo sound, but it makes the sound on the Yamaha KB fuller and more even. No parts are missing now
Hope this helps.
Eric