Yes, you can unfuse Kyurem.
In OR/AS, once you have reached post-game content, you must access Kyurem's Mirage Spot while Soaring on Latios/Latias. This requires you have both Reshiram and Zekrom in your party. Once there, in addition to being able to challenge Kyurem to catch it, you can use the Itemfinder to locate the DNA Splicers on the ground in the cave.
Once you have the DNA Splicers, simply use them on your fused Kyurem with space in your party. It will pop apart, giving you your ordinary Kyurem and either Reshiram or Zekrom, depending on which one was fused.