I was wondering if I will be able to find Kyurem in my Alpha Sapphire copy (where I already have caught Zekrom) if I use a Reshiram I'd transfer from White 2. Or will Kyurem only appear if both Reshiram and Zekrom were caught in a ORAS game?
You can use Pokemon obtained in older games to get a certain Legendary Pokemon, which also includes Reshiram/Zekrom.
Source: Tested this myself with a Gen V Event Zekrom and a caught ORAS Reshiram and was still allowed to unlock Kyurem.