Technically speaking, I feel like it will be a challenge to require players to buy both.

It was easy with the base game because you either can play on PS4 or you cant. However, with the expansion, what if you log out in a new zone as a Machinist on PC and then attempt to log in on your PS4. They can't simply deny you the ability to log in, but then what? Auto switch to a different class? What if your inventory and armory chest are full? What happens to the gear? The issues really compound when it comes to this.