Pretty much the way you are doing it - allowing them to sniff each other under the door for a few days - a week if I could manage it. It's always tough in the beginning, even after that "smell adjustment" period. You can still expect hissing and stuff. I would recommend getting this stuff called Feliway - it emits a feline pheromone that seems to chill them out. It plugs into the wall like those glade plug ins. It's very expensive, but effective - about $25 for one. You can get them PetSmart, PetCo and those kinds of places. We bought two when we first moved - one for upstairs and one for downstairs - to help keep everyone chilled out during the adjustment period. I do believe it helped a lot!