They probably didn't start fighting immediately because the first order of business was to get used to their surroundings. With that out of the way, they can now decide who's "top dog". It's unlikely they'll actually do damage to each other, but not out of the question, either. The fighting will eventually decrease or even cease altogether. They may never be friends, however. But even if they aren't, I'm still of the opinion that they are still happier with another cat in the house than alone, if they are with working "parents" and have no humans or other animals in the house.