hmmmm....tough one.

The first step should be a vet visit.

Since I do not know how old these cats are, or their background before you adopted them, or even if they are spayed, my ideas are general.

#1- Two females will often NOT get along together. It could be as simple as that.
#2- Abby may just need to be an "only cat". Which might mean that you would need to return her in hopes of adopting a more social, cat-friendly cat.
#3- If one or both are not spayed these types of problems are often exaggerated. Spaying can cure a lot of it.
#4- Products (like feliway) or medication (like amitripylin) could help ease stress and tension and allow bonding between the two of them.

Normally I would reassure you that all will settle down withtime, but with the inappripriate peeing and pooping, the strategy might be different.