From your answer, I take it you are not looking for one. It is not what you described. It is called also auto-accompaniment where one plays a chord on the right keyboard, and short backup sequence of tracks is played matching the chord you played. The reason I asked is because these keyboards usually have good emulation of acoustic sounds and some are affordable as new. I was going to recommend PSR 2100. But again they it does not synth, but it does have good midi functionality and 16 track sequencer.