He holds them at gunpoint, forces them to to do his bidding, and ultimately pulls out a knife and begins stabbing. It would take seconds to switch between weapons, putting his gun back in his pocket. He’s not holding them at the same time, and there is absolutely no difficulty there.
Stabbing is incredibly common in sexually motivated crimes. So he stabs them so as to incapacitate them, and slits their throats as the coup de grace. I think it’s more probable than not that he used the same weapon, but it’s totally within the realm of possibility that he switched to a box cutter for this.
I say this because if he had time to place branches on top of their bodies, then he had the time to also slit their throats with another weapon.
Law enforcement apparently had interest in different types of weapons when they served that search warrant, which is why I think there’s a chance he did use two.
We can’t be all that far away from hearing from the ME. Maybe even today.