Why didn't Fuyumi and Hydra Bell fuse like their mothers did? From what I understand, When two opposite individuals (one from the human world and one from the demon world) touch each other, they are fused together to create one new individual that is made of the two of them. 

So if Fuyumi and Hydra Bell are both the daughters of Nyen (Nene), why they didn't fuse when they touched each other?
The straightforward answer to your question is that Fuyumi and Bell aren't doppelgangers ("opposites") for each other, and so there's no reason one should expect that they would fuse with each other. 

Sure, their mothers were doppelgangers, but that doesn't mean that their children - Fuyumi and Bell - should also be. For one thing, they don't look very much like one another, whereas their mothers (pre-fusion) totally did: