Yes, you can fuse Reshiram with Kyurem in Black 2, however this can only be done when you defeat the Elite 4, battle N, catch his Zekrom and catch Kyurem. When you go back to the Giant Chasm, Kyurem will be at lv.70.
Then you can use the DNA-Splicers to fuse Zekrom/Reshiram with Kyurem
Note
This can only be done after you have defeated the E4