Bernie I to labor over the same thought; There is a right and a left but no combination output. I used a Y- connector to correct the problem. As Eric said, you lose part or all of an instrument.

All my keyboards have a combining output L/R. Which one of your keyboards does not? Is it Yamaha?

I gave up on stereo a long time ago. My Opinion: the people in the center enjoy the stereo sound. The ones on either side do not get the full sound. I often, when needed, edited the panning of each instrument and placed all the instruments in the center to overcome the stereo effect. The Y- cable seems much easier.

John C.