I built that anchor and it took me 10 minutes to get it right in the comfort of my own living room. I'm sure I could get it down to 5 minutes, but lordy, it's a PITA.
So the reason for the two biners is apparently to guard against the total failure of one side of the anchor, in which case a single biner on both strands would slide off. But why not just clip both strands using one biner and put a twist in one of the strands, same as for a sliding x?