I going to explain it as I possibly can,
1. Go to Performance Mode
2. Pick any patch
3. Push the F2 key (should be called Voices)
4. Usually what I do for the top that's for the left side and the second one is for the piano so to change the sounds, hit category and go to bass and pick a sound then hit Enter.
5. Do the same for the piano.
6. If you have extra voices in that patch, you can delete them unless you want to do layering...and if you do that do the same steps in step 5).
7. To physically split the board, on the bass sound, you press and HOLD the SF5 button (Limit H) and press the key where you want the highest key to be set at. Then press and HOLD the SF4 (Limit L) and hit the low A key. For the piano sound, press and HOLD the SF$ (Limit L) and press the key you want to start the piano split. (For example, if you end the bass at C4, then you start the piano Lower limit at C#4). To be sure that it's including the keys at the end, HOLD the SF5 button (Limit H) and hit the last C key.
Hope that explains it!
I have a MO but should be the same as rhe ES6