I have two adult male cats. One is 2-3 years old and the other is almost 12 years old. For the majority of their lives they haven't lived with any other cats but seem to tolerate dogs well. I need to introduce them to one another (I will be bringing the 12 year old into the house the 2-3 year old lives in). Does anyone have any advice on how to bring them together without a lot of fighting?