This really is a monster of sorts: a piano 164 keys and four pedals. And what's more it's the only one of its kind.
It does not add a new sonority. The double-keyboard piano sounds like a piano, but with fuller chords and denser harmonies. Unlike an organ with additional stops and pipes or a harpsichord with separate strings for the second manual, the double-keyboard piano still has only one set of hammers and strings.
Read more all about it here.
And a nice video clip let's you see it in action here. Pianist Christopher Taylor discusses the instrument and its possibilities.