The manual doesn't appear to say anything about how to select the tracks (MIDI channels) you want to record to. It does say you can play along with either a song or an accompaniment, and that you can record yourself playing along with a song or accompaniment-- but it isn't clear whether you can add more tracks to the recording, or whether recording yourself playing along with a song will simply re-record (overwrite) the tracks you'd previously recorded. You could try the following and see if it lets you add more tracks:
(1) Set up your tone(s) for the main parts of your performance, including layering two tones for the right hand and/or splitting the keyboard if desired. (I'm assuming you can record layered tones or split tones, but I'm not certain.)
(2) Select the accompaniment you want to play along with.
(3) Record yourself playing along with the accompaniment.
(4) Save the recording to a song file or SMF.
(5) Set up your tone(s) for the part(s) you want to add to your recording.
(6) Select the song file you just recorded, then press the plus ("+") button (see the memo in the upper right quadrant of page 26).
(7) Record yourself playing along with the song.
(8) Save the recording.
Either step 7 will record additional tracks to the song without erasing your previous tracks, or it will overwrite the tracks you'd previously recorded.
Of course, if the keyboard doesn't provide a way to do this, you can always use a DAW to record as many tracks as you want.