I am not even going to try to figure out the logistics of your demon world.  Two halves make a whole is what I got out of that chapter, I am going to accept the math and move on.

Great solution, hope it works

It's a good plan. But will Willow let it happen? She's done everything she can to stop them from finding the cure. If she finds out about this before they get it done bad things are going to happen. I feel sorry for Eric. He's stuck. He isn't fully human and he isn't a full demon either. He needs to go with his only remaining parent to have any kind of life.