Cat's are territorial and have a pecking order. By removing them from their normal environment you have upset the previous "agreement" that they had. They are like new room mates that have to work things out. The cat that stayed out is most likely the more confidant/aggressive individual and will try to assert its dominance every time he/she reenters the home. It can take 2-4 weeks for a cat to adapt to a new cat in the house and I would expect that this rule of thumb could apply to this situation as well. It is generally recommended to put the cats where they can see or smell each other, but can't make contact for the first couple of days. Since these two already know each other, I would try not to leave them alone with each other until the hissing stops. (Probably a few days to a week) In the end, they will have to work it out. I have 4 cats and they have been together for over 5 years. Yet they will occasionally hiss at each other and then I will find them cuddled up with each other an hour later (like siblings having a spat). Good Luck
