I agree with the second response, when multiple characters are involved in doing hand-ins for a single quest.
An NPC has a "cache" of items which it will retain until either the quest is completed (all hand-ins are done), or it respawns (zone reset or respawn).
As a side note, it is also possible to MQ with two characters from the same account, and it works in the same way. In the JBoots example, say you get the ring off of the ancient cyclops with your high level druid, and you'd rather have the boots on your slow warrior. Make your warrior has enough money, and put him in Rathe Mountains on the path of Hasten Bootstrutter. Log in your druid (who has the ring), go get the item off of the shadowed men, and head to RM. Find Hasten and hail him (hopefully near where your warrior is camped out) and hand him the ring and the shadowed rapier, then switch to your warrior and hand in the 3250 gold. Since your warrior will be the last to hand in an item, he will then get the boots.
Note of caution: A former guild mate once tried this, and someone had already handed in the money, so when he did the first two hand-ins with his necro, he got the item himself. I thought it was hysterical, he didn't.