Why does one of your cats turn to the other, and attack, in response to something outside? This lashing out is called redirected aggression, and is a common problem in multi-cat households. It might be present in your household, even if your cats are indoor-only.
Dr. Francine Rattner, in the Capital-Gazette's "Ask a Vet" column, describes how redirected aggression is much more of a problem for cats than for dogs. With dogs, according to Dr. Rattner, they might attack each other in response to a dog on the other side of their fence, but they "forgive and forget" pretty quickly. She says the same is not true of cats. Redirected aggression can be a source of long-term conflict in multi-cat households.
The first thing you have to do is identify your cats' trigger. Oftentimes, it will be seeing a strange cat outside a window. As hard as it might be, you should restrict access to windows where they might see the strange cat. Dr. Rattner says shutting the drapes may be sufficient, however, it might also be a good idea to find out if the cat belongs to anybody in your neighborhood. If so, you could consider talking to them about keeping their cat in their own yard.
You might also consider some "cat repellent" to keep strange cats out of your yard. Rosemary, hawthorn, rue herb, and lavender all give off scents that cats don't like. Depending on where you live, you can plant these around your garden and near your house to help keep these cats away. Also, if you have bare spots in your garden, where cats like to use the dirt as a litter box, you can use something like river rock or lava rock as a ground cover, so they can't dig nearly as easily.
This, alone, won't solve the problem, though. Removing the trigger is only a part of everything you need to do. Dr. Rattner says one way you can stop your cats from attacking each other is to separate them, and then slowly reintroduce them. The Cat Coach, from the website The Cat Coach, agrees. To make your cats become friends again, separate them, allow them to calm down in a dark, quiet place, and keep them separated for awhile. Then begin reintroducing them to each other.
One thing you can do to keep stress as minimal as possible is to use a pheromone diffuser in the room where you're reintroducing them. Feliway has such a diffuser, and you can find it at Petsmart. The pheromones mimic the friendly pheromones found in cats' cheeks and foreheads, and helps to keep them calm.
The cats need to associate each other with friendliness and warmth, not with fear and anger. That is what will ultimately stop the aggression. If you're having problems with aggression, redirected or otherwise, and you don't know what to do, contacting an animal behaviorist might be a good idea.