The best way to be sure that any two sweet critters including kittens will be friends is to get them at the same time so, just like kids, human kids, bond early in life together. Otherwise, it is best to take it gradually. Don't just thrust them together but while you continue to give the first one the normal amount of attention, introduce the new one a few minutes at a time. They can become good friends. I recently introduced a pet cottontail rabbit to my 2 dogs ages 6 & 7, and it worked. Now they play and even lie side by side kindly.