I will assume you are talking about devices that carry ethernet over power wires, this is kinda in the wrong forum category.
If you were to run 2 pair of device they will interfere.
It depends on what you need to do. If you want you can add powerline devices into the first network and since they all talk they will not mess each other up. They do though share the same total pool of bandwidth. You do this by changing the encryption keys they are using so they are the same. The user manual explains how to do this on most models.
If you need the networks to be separate there is no simple way to do it with just the powerline units. If you just run them as 2 pair it will be kinda like using wifi on the same channel as your neighbors.