Hello there. Here we go again in attempts to contribute. I kind of got excited when this happened because hopefully someone can find a more useful way to utilize this exploit.
As you know, in Northrend the Zones have mini-instances within their respective zones. For example you do a quest, and the zone itself will change for you, reflecting how far you have progressed in a specific questline.
Well while 3boxing in Icecrown I got ahead of myself and accidentally finished some quests on 2 of the toons but not on another. Finishing those quests caused The Shadowvault to turn Friendly with ebon hold npc's in it, while the toon that didnt do the quest, had hostile npc's in it still.
We were in the same zone, same time, same area, just the "area instance" was diff for the two toons, due to the quest completion.
As you can see, we are grouped, in the same place, just diff "instances" of the same zone. Well as I was completing the quest, killing mobs, i noticed that the other 2 toons in the "friendly" instance were still getting xp, WHILE getting rest XP bonus and accruing rest xp at the same time. This in a way is sort of a "mini-infinate rest xp" type of situation.
havent tested the extent of how much you can interact with each other between the two instances. But it has the potential of being a good PL tool in many of the other zones with this type of in zone instance. Or im sure someone can find something creative to do with this.
Hope this is useful to someone.