Who are Katie and Kaitlin?
Katie and Kaitlin are special visitors that might appear after you have visited another player's town with Nintendo WFC. Kaitlin is the mother cat, and Katie is the daughter.
How to find out where to go
If Kaitlin is in your town, she will tell you the name of the town where she got separated from Katie, as well as the name of the player who lives in that town. But if Katie is in your town, you'll just have to hope that the person who has Kaitlin will contact you about meeting up so that you can take Katie back to her Mom.
How to reunite Katie and Kaitlin
Whoever has Katie (the daughter) will need to talk to her, then walk her into the town gate. Be sure to walk slowly so that Katie can keep up with you. She is very young and small, so she can't walk quickly. If you go too quickly, she might trip and fall, and you'll have to go back and talk to her to get you to follow again.
Once you're in the gate, talk to Copper to open the gate and visit the town where Kaitlin (the mother) is waiting. When you open the gate successfully, you will see a cut scene of Katie and Kaitlin being reunited. Be sure to save the game after this happens to prevent having to do it all over again if something goes wrong.
Do I get a reward?
Once you reunite the cats, you get a present in the mail from them. These are special items that you can only get by reuniting Katie and Kaitlin. Here is a list of all Katie and Kaitlin furniture.
What if I don't have access to my friend's town anymore?
If you are unable to visit your friend's town anymore and you have Katie or Kaitlin, you will just have to wait for them to go away. I'm not sure how long it takes, but they do leave after a while.