trade them in or use a cheat code.
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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.
trade a Pokemon from another game that is holding exp share to the game that you want it on.
Level 10.
Nope
An oak aide in a big building east of fuchsia city has exp share you need to record 50 Pokemon in your pokedex and show him that, he gives you exp share for your accomplishment.