What about professional musicians, who have to concentrate on far many more things than two at once? Organists, in addition to playing anything up to five keyboard manuals with their hands and one with their feet (simultaneously reading anything up to twelve lines of music, though in practice usually never more than five), have to listen to a choir and/or congregation, watch a conductor, and read the music, all at the same time. Some of them can even sing competently one line whilst doing so!
Whilst I can accept that it is very difficult to consciously concentrate on more than two things at once, somehow some people can train their subconscious into doing so -- when sight-reading music, I experience a lovely sensation, almost as if my brain is being "split" down the middle -- if I concentrate for too long, I start to develop a headache and feel exceptionally exhausted. It is a most wonderful feeling, and nothing else in the world quite comes close (although doing some rewarding mathematics isn't far behind). I would not be surprised if it were possible to find many more examples of people concentrating on more than two things at once, "simply" through getting other bits of their brain to do the dirty work. Juggling on a unicycle while jumping over a skipping rope, anyone?