You have to trade the other item from a trade, but it's easier to get a friend to raise the other Clamperl evolution and trade it to you.
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I'm pretty sure that you have to trade it between emerald, ruby, and/or sapphire. You can't get both with one game (I think). You can get them both in Pearl and Diamond, though.
To get Gorebyss or Huntail you have to go to the Abandoned Ship and there you see a scientist who tells you that he is looking for a Scanner. You have to find the scanner and go back to him and he tells you to give it to Capt.Stern. Fly back to Slateport and give the scanner to Capt.Stern and he will give you a choice of the Deepseascale and the Deepseatooth. You have to choose one. Both increase the Sp.Ack and Sp.Def of Clamperl. If you give Clamperl the item and trade it. It depends on what item you take from the Capt. if you get Gorebyss or Huntail. Both are evenly matched.
You have to catch one in emerald after the elite four in Pokemon emerald and you have to trade the other lati from Pokemon sapphire or ruby.
You can get one from a Carvanha, or Sharpedo. Both have a 5% chance of holding one. Good luck, get catching!
have Pokemon pearl and emerald both in the ds. it will say migrate from emerald on the Pokemon pearl screen. that`s all you need to know.