(Just looked them up, never heard of them before, interesting! I'll have to find some recordings)
From what it says on Wikipedia, it's played as a duet - i.e. the music you play is still in time with itself; your part is in phase with the other part.
Still hell to play, yeah, but it would be pretty much impossible if the sound from your own piano were to come at you with an audible delay. It would be easier to play if you were deaf - at least then nothing would interfere with the rhythm in your head.