I have a PSR-S910. I have 4 registrations with the left hand as a grand piano. This is fine. I added 2 more registrations with the left hand as a bass. However, now all the registrations have a left hand bass. How do you separate the left hand for the first 4 registrations to stay as a piano on the left hand and the last two stay as a bass on the left hand?

Make sure of your parameters when you save the registration.