Now navigation system on new aircrafts are so accurate, that aircrafts fly always excactly on the centerline of the airway With a few inches precision, at the exact altitude.
So if by misunderstanding both aircrafts fly the same airways at the same alt but in opposite directions (providing tcas doesn't work) there is a 100% chance they will touch each other.
I heard that on new airbus when flying to remote aeras such Africa, the navigation system is randomly selecting a track not exactly on the airways centerline but within a few Nm offset to reduce the risk of such collisions.
I am exact or wrong, someone could confirm what i said.