For this cheat you'll need two people, Legendary Heros Souls, and an item you can drop from weapons to other souls etc.
Summon a blue phantom or black phantom. Have the phantom drop the items. Then, have him either quit game with the PS button or pull his ethernet cable out (not recommended). Summon him back in and have him drop more of the items and leave again. Each time he does this he'll keep his share of the items like he never dropped them and you can collect the copies.
You can do this with Legendary Hero's/Soldier's Souls. You can only do this with items you can drop. Not boss souls, sadly.