I think your question was - does a keyboard exist ....
The answer is that almost any instrument with accompaniment can turn the accompaniment off.
But if you turn the accompaniment off then you generally will get only the notes you play. If you are expecting the keyboard to play a full chord from your two notes, when accompaniment is switched off, then I am not sure what keyboards do that. And what do you mean by 'play a chord'? Do you mean a left hand chord?? And will you be playing melody notes ??
I don't think its that clear what you are trying to do, so maybe you can explain more fully.
Mike