Hi Arie, Not at all uncommon. In the New Complexity scores that I engrave for a colleague composer we have this continuously, in the piano music. It's actually a fairly standard modernist notation, and true enough indicates the distribution of hands but also that this is a single dyadic chord event, not multiple voices. I didn't answer yet as we tend to have the noteheads on the same side, but for the case with the notehead on the opposite, you now have lots o good solutions. So don;t think this is just Bartok.
Andrew
