Also, I'm not sure if I'm interpreting things properly. It seems to me that the Bolt Thrower is a unit, and the Crew are a separate unit, but that the bolt thrower only works if models from a Crew unit are nearby. Most of the time this won't matter - you can split shooting attacks however you like, and it's possible to Charge one unit and Pile In to another.
However, it does mean that if one crewman dies, the other can fail a Battleshock test and leave the machine uncrewed. Likewise, it looks like you could, if multiple machines are close enough, have one pair of crewmen operate a bolt thrower each (although it doesn't really make any difference) or have crew from one destroyed machine wander over and man an uncrewed one.
Or I'm overthinking things.