Kawai CN390 Owner's Manual page 21

If you want to start recording the second part from the beginning of the
song, just play the keyboard. You will hear the first part playing back
while you record the second part.
If you would like to play back the first part and then start recording the
second part from a certain point later in the song, press the PLAY/STOP
button instead of playing the keyboard to start playback of the first part
and then begin recording the second part from any point you like.
When you are finished recording, press the PLAY/STOP button
again.
Hold down the PLAY/STOP button and look at the Part buttons. The LED
indicators for both parts will be on, indicating that you have successfully
recorded two tracks. Press the PLAY/STOP button to listen to what you
have recorded. You should hear both parts.
Both parts will automatically be set to play back. If you do not hear both
parts playing, you may have made a mistake when recording.
If your song is not playing back properly, carefully reread the previous
section to make sure you're recording properly. You can use the LED
indicators (see Song Status and LED indicator section) to help determine
if you have a problem.
Here are some basic things to check for:
1) If you only hear the second part, you may have accidentally recorded
over the first part. In this case, just rerecord the first part you played
after selecting the "empty" part to record (in this case it will probably be
part 2).
2) Did you record one of the parts in a different song? In this case record
the missing part in the appropriate song.
TUTORIAL: USING THE RECORDER 21