The Cross Beam system is a special case of triggering. It is not
required for most Shutter-Beam applications. If your subject is baited
or forced to one location of the beam, (like a bullet, or a nesting
bird) you can focus on that location and be fairly sure that your
subject is not going to enter the IR Beam outside the cameras field of
view. Two beams are not necessarily required unless you can not
tolerate the occasional missed shot.
is an example of when you might need to use two Shutter-Beams. They
fly very fast and in erratic patterns. The field of view, and depth of
focus is very small. Triggering has to be in the middle of the
frame or the insect could easily fly outside of the picture (missed
shot). The Cross beam application increases your odds in this case,
but you don't have to use the Cross Beam system to get the shot.