Quests ARE stackable. UPDATED 3/05
 

You have to be in a party for it to work. You don't have to be anywhere near anyone, nor does the people in the party have to have the same quests as you. Just as long as you're in any party regardless of distance from any of the members, your quests will be stackable.

Basic Quests
2 Kill Quests: stackable
1 Kill quest 1 Drop quest: stackable
2 Drop quests: unconfirmed

Transcendental/Heroic/Fate/Trial with Basic quests
2 Kill Quests: unconfirmed
1 Kill 1 Drop quest: unconfirmed
2 Drop Quests: Unstackable

Is it only for the bulletin board ones? Cause I have one Fate quest and 3 bulletin board quests... all with the spider but it only counts spiders for the fate quest. x__x

So far, its stacked my bulletin board quests. I'll do another test on Fate/Transendental/Trial quests when I'm given the oppurtunity.

*Just did two drop quests, one of which was a heroic and the other was a basic quest. They didn't stack, I can assume that it wont work for kill/kill or kill/drop quests but I have a feeling drop quests are just special like that. They might not give two drops at a time from one mob, as opposed to getting a drop from a mob and taking credit for the kill as well.