I agree with Carey. Dwarf Angels can often be territorial. If they are in a smaller tank, they can easily decide that the whole tank is their territory. I cringe when I see sites saying dwarfs are okay in 30-40g tanks. I think 75g is a good minimum, but even that depends on the personality of the fish you get. I had a flame in a 300 years ago and it claimed the left third of the tank. Any of the much larger fish came into that area, it would chase them.
Keep an eye on them and if it continues after a day or two, I would consider taking one or the other back.