While playing with a friend (or a second controller), find a shopkeeper and have the second player (person without the flag) pick it up. Then, using the main character (person with the flag), continue to finish the level while the other player sits in the shop holding the desired item. You do have to complete the level before the person's time runs out or he will die and you won't get the item. Also, if you pick up any item that you can hold onto (jetpack, cape, mattock) it will duplicate and both you and the other player can have that item. All other items will go to the player that held them in the shop. Just kill them off and take them for yourself if you so choose.