1. Does "just as" mean simultaneous action of "asked him" and "he noticed" or sequential one? I think the former.
2. Does "work two shifts" mean "2 times of 8 hours(1shift) = 16hours" or "the half of 24 hours = 12 hours"?

rp106)...Thinking that the boy had gotten separated from his parents, the man asked him what the problem was, just as he noticed the boy was holding a hundred dollar bill. The boy explained that he wasn't lost or homeless, but that his mother worked two shifts at the hospital and had no time to shop for Christmas gifts...