Aside from the answer mentioned, there's also the Pokemon/Item Cloning Glitch in Pokemon Emerald. I don't about the legality of this glitch, but I can vouch for the fact that it works. However, you need to have beaten the Elite 4 and Champion at least once and gain access to the Battle Frontier.
- In the Battle Frontier, go to the Battle Tower
- Deposit your Pokemon (holding the Exp. Share) into the PC in the right
- Save your game after logging off from the PC.
- Withdraw your Pokemon (holding the Exp. Share) after saving
- Talk to the lady nearest to the PC and enter either of the levels
- Once you've chosen your 2 Pokemon (any) following which there will be two pauses. After the second pause, when the lady asks again if you want to save, you can soft reset the game
- When you restart the game, the Pokemon (and Exp Share) will be cloned; there will be one copy in your Party, and the other in the PC where you deposited it before it saving the first time.
Congrats, you now have two Exp. Shares.
If you're still confused, this video is a straightforward guide.
P.S. Your mileage may vary. If you have an authentic cartridge for Emerald that you do not want to compromise, or feel it is unethical to use glitches, then the choice is entirely yours. The option's there, and I figured I'd let you know.