It sounds like you have not introduced the cats properly, or the new can would not be “always hiding except when she eats”. Cats are very oriented to routine and don’t like change very much. Yes, two cats can get used to one another, but the introductions need to be done in a very specific manner.
This means: Put new kitty in a separate room such as a bedroom, computer room, bathroom, etc with her litter box, food and water. If it is a room with glass doors, even better since the two cats can see one another through the glass. The new kitty is quite scared because:
-She is in a new house.
-She is likely missing her old owners.
-She does not know your current cat.
-She does not know if someone or something will jump out at her and harm her.
-She does not know you or your family.
The smaller area gives the new cat time to adjust to being in a new place. In a smaller area, she has less to contend with and will feel more in control; it is easier for her to learn what is around her and what is safe and unsafe. In this controlled area, should be less frightened and apprehensive. This also gives your old cat time to adjust to her scent and presence, and he will be much more likely to accept her.
It us usually best to keep the newcomer in the separate room for up to one month. By this time, she and the current cat should adjust to one another. At that point, you will know if they are getting along, and you can be more able to make the decision as to whether you can then progress in keeping her.
If, of course, you just don’t want to go through this and the place you got her from is willing to take her back, then you could do that, as well.
ALSO: BOTH CATS NEED TO BE NEUTERED OR SPAYED. If they are not altered, you will have all sorts of problems, and especially since many females will reject an unaltered male cat and feel very threatened; some will even attack the unaltered male cat if he advances without her “permission”.
AGAIN: You have unfortunately and unknowingly violated the “how to introduce two cats to one another” rule. You can now begin again and hope they will adjust properly.