1. It is possible that someone could test negative for both B1 alleles (gene pieces) and be positive on the A1 gene.
2. The main reason for looking at both A1 and B1 is to avoid the possiblity of false negative findings. (i.e. B1 is negative and A1 is positive). If someone has already been identified as positive B1, the significance of knowing the A1 gene alleles has more to do with determining family risk. The more genetic pieces, the greater likelihood they will be passed down to children, the greater odds that the parents and siblings could be gluten sensitive.
Hope that helps clarify,
All the best,