Cats reuniting after almost 2 years... can they be friends again?

OK I have two cats in my house now: One small shy young female and one much older (and starting to get a little cranky) male. They used to live together almost two years ago, but due to some unforseen circumstances, they were separated for two years and now I have to take the female in again. They seem to recognize each other, but one gets too close and the other hisses. Now the poor little female won't leave my side. I tried to make them share treats, play with the same string, share the same catnip scratchy-board, and even found some marbles that the female and the male used to play with to get them to like each other again. Everything works up to a point, and then they hiss at each other and won't share the same room until I bring one in. I have errands to run and I don't feel comfortable leaving the house if all I come home to is two kitties who have battle scars from fighting all day. What do I do??