After watching the crime scene flow it's entirely possible the girls saw BG get out of a car toward the end of their trek across the bridge - parked on the drive that runs past the bridge and underneath it even past the bridge short of the house. He could have walked past them to see if anyone else was coming down the bridge and then turned around alerting them to snap a pic as he came back to the end toward them and the car. This would have given him observance of both exits. Then could have ordered them down the hill to his car but then they probably bolted and he gave chase catching up when they got to the water. They probably stalled not wanting to cross and hesitated and that's probably where he attacked them catching up to them. Crossing the creek may have been to wash DNA or maybe the attack continued across as they may have felt it was their only way out. Then I'm guessing he probably doubled back to the car and took off. So, basically a botched abduction that turned into a double homicide. IMO
Also, in a two person scenario there could have been one other person in the car to help give chase and control a second victim.