You need at least one memory card and two analog controllers for this trick to work and is only valid for the Normal (non-Internet) Campaign.

The basic premise is that you can import your own character, then dump the clone's equipment on the ground so your original character can snatch them up and sell (or use) them. To do this, press start after saving, then import your character from that save into the second player spot. you now have the same equipment and armor and weapons, etc. The game tends to delete items if they are left on the ground, so it's best for any items you intend to transfer be on the original character before you add or delete a cloned character. this works in both 2 player or 1 player mode.