Not if the 2nd person(s) were not on the bridge. One of my theories about the voice is that while Libby was filming BG on the bridge 80 ft away, the voice came from near or behind her, from someone closer to the phone. (It happens all the time we I am taking a video of my son playing baseball.) I think BG's role as we see him on the bridge was rounding them up, blocking their escape and that later he participated in murdering them. They only needed one person to go out on the bridge and force them off.
Which could have been the infamous unknown sleeve in the one pic.