Unless you cheat, you can only obtain Reshiram in Pokemon black. You will need to trade to Pokemon black if you have white for zekrom to be in Pokemon black.
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If you are talking about the 2 mascot Pokemon then you have the following: Reshiram - The "Vast" Pokemon. Reshiram is Fire/Dragon type and is only catchable in Pokemon Black. Zekrom - The "Deep" Pokemon. Zekrom is Electric/Dragon type and is only catchable in Pokemon White.
In Pokemon Black you will encounter the white Pokemon Reshiram, and in Pokemon White You will catch encounter the black Pokemon Zekrom. Unfortunately, you cannot catch both in one of those games.However, you can obtain both Zekrom and Reshiram in one game by trading one over to the other game. That is the only way how you can get both in a single game.
You can catch Zekrom in Pokemon White and Reshiram in Pokemon Black. They are version exclusive legendaries so the only ways you could get both is through hacks or trading with someone.
The only Pokemon that you can fuse is Kyurem with either Reshiram or Zekrom however you will need to use the DNA Splicers item in order to activate the fusion.
Yes you can fuse Pokémon however this is only possible in Pokémon Black 2 and Pokémon White 2. Using the DNA Splicers, you can fuse Kyurem and Zekrom to make Black Kyurem and Kyurem and Reshiram to make White Kyurem.