You can get the two Exp. Shares in Pokemon Black and White these ways:
1.) Go to Iccirus City. Go into the Pokemon Fan Club and show your starter Pokemon to the guy in the middle of the room. If your friendship is high, he'll give you an Exp. Share.
2.) Go to Castelia City. Enter the Battle Insitute. Battle and beat everyone, and on the highest floor, if you beat everyone, you can beat the director and he'll give you an Exp. Share.
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trade a Pokemon from another game that is holding exp share to the game that you want it on.
Nope
You can not get two xp shares in Pokémon Diamond (that I know of). You can trade exp shares from other Pokémon games to your Pokémon Diamond Version. That is what I did.
You can either get it in the game, which apprently you CAN in pokemon black and white. And just as the previous contributor said, You can trade with your friend and then BAM! You get an experience share!
Use the experience share item and attach it to a pokemon that you want to gain exp. Also if you start with the weak pokemon and swap out for a stronger pokemon, then half the exp goes to each pokemon