Both the Roland attelier Organ as wel as the Yamaha Stagea have a solo voice register..

Solo voices work a little different from normal voices as you can choose to use them on top of the normal voices, but only sound on the top not (of the right hand) or thelast note played..

I have been wondering why this never found its way to arranger keyboards ( nor to the Wersi organs)

I was at music store today and got to toy with a use Roland atelier, and really loved this feature..