I had a similar situation myself with the shelter telling me 2 cats were bonded, but that was not the case when we got home! they played together fine at the shelter actually, although I never saw them really cuddling together, so I was dubious. but I'm a sucker for a cat who needs a home and we ended up adopting both because after we took the first one home, the shelter said the one left behind was really acting "out of sorts."
anyway, we survived. we did end up spending lots of time preventing skirmishes, I'm afraid. mainly because I was very protective of the older one - she was 15 with really bad arthritis, gentle as can be, and completely declawed. she had also had a really hard life being bullied by her sister before being rescued. the younger one was a young, healthy, fully clawed 7 year old boy. sweet but a troublemaker.
I think what was going on was a dominance/pecking order thing. we already had an 11 year old arthritic cat (Angel) and adopted the 15 year old (Lucy) as a companion for her. Totally wrong decision on that one, too, btw. Angel did NOT want a buddy at all. live and learn!
so basically , Angel was #1, Lucy was #2, and Frankie the young buck was low man on the totem pole. I think he did not like that at all. angel put him in his place IMMEDIATELY by scaring the crap out of him. kinda funny as I look back, cuz Angel was much smaller and older than him and had arthritis and was also front declawed, but she was a banshee when she hissed at him. Frankie would run away crying like a baby every time. he knew not to mess with her. so, unfortunately, I think he decided to try to dominate Lucy instead.
sorry for the long story, but just letting you know it might be hard work for you to keep watching them if they keep fighting. altho, i agree with an earlier post that their skirmishes aren't too serious right now. the more serious part might be the pooping in different areas. you may not want to deal with that if it gets any worse due to them fighting for dominance.
I think the really easy thing (heartbreaking at the same time unfortunately) may be to do what Dizzy's Dad mentioned - rehoming one of them. if the store actually WILL extend your trial period so you can see if things improve, that would be great. if you do rehome, definitely return whomever you decide to the same store and let them know the story so they can do their best to find her a suitable home. they have the rest of her history so it is best to return her to them. they may need to insist she be the ONLY cat in the future -otherwise she could end up being returned again and again, which would be truly traumatic for her.
I think after being in a hoarding situation, then experiencing what it's like to have your own space, then being forced to share it with someone else again might be tough for a cat. they get the taste of freedom, space, and privacy and don't want to give it up again! wait a minute, that sounds like me! ha ha!
good luck grandma! I do hope it works out and I'm sorry if it doesn't with both of them at home.