I was looking at crossing zeros when I was writing my simulator and I decided
they were impossible when I was looking at the pattern (2x,0x). In this 1 ball
pattern, a ball is thrown from the left hand to land in the right two beats
later and then thrown again two beats later from the left hand. To me it
didn't really make sense that a ball could have no flight time, hence why
zeros typically don't involve a throw. As Alex said you can look at them as 1s
or 2xs but then (in my opinion) it's a 1 or 2x and you have to change the
other numbers to work with that. So (2x,0x) if you consider the 0x a 2x
becomes something like (3,0)(0,0)!(0,1)!
Another way to look at it is (2x,0x) is a one ball pattern yet two balls are
being thrown simultaneously.
Just my $0.02