Controlling first impressions and keeping the cats apart but able to see each other are two steps that can ease the initial introduction between two cats. Allow each cat her own food dish, water bowl, scratching post and litter box. Keep the cats apart, but feed them at a centralized location separated by a clear door to show the cats coming together is a positive experience.
After a few days in the new environment, switch the cats' locations to enable them to smell each others' scents. Play with the cats by the shared door, encouraging them to paw underneath the door for play. After one week of separation, if there are no signs of aggression, introduce the cats to one another, and begin allowing them to spend time together on their own. Play with the cats near the barrier where they were separated to encourage them to move about freely.
Try to bring the cats together at calm times, such as after a meal or intense play, to minimize fighting. Using a squirt bottle filled with water is a good way to stop fighting if it becomes a problem. Giving cats plenty of space with both perches up high and hiding spaces down low can help cats adjust and enjoy their environment.