How do 2 guests with swords keep from killing each other in RomanWorld?

I would guess that it would be the composite materials the swords are made from.

To make a great film you need three things - the script, the script and the script -Alfred Hitchcock

Perhaps the robots in the Medieval and Roman Worlds have different programming to those in Westworld. Their safety feature would be not to lunge with the sword at a guest? IIRC the guest who was killed by the Black Knight seemed shocked by how aggressive it was being towards him and that was the first indication the control staff had that something was wrong.