Fusing them means to combine them. I'll explain.
You need to item DNA Link Pin, which is found after catching Kyurem in Giant Chasm. After fusing them, use the item to unfuse them.
Once again, the DNA pin is needed. use the item to fuse your Kyurem with Zekrom to form Black Kyurem in Black 2, or Kyurem with Reshiram to form White Kyurem in White 2.