So I have two accounts (is that legal? lol) and the second account is linked to my boyfriend's main account. Triple xp is awesome, and what makes it the most awesome is that 1. it doesn't use up your rested xp, and 2. running instances is the fastest way to level up your linked toons. That being said, in the past couple of months, I've been "running" low level instances and helping these linked toons get to 40 pretty quickly.
This is a quick guide for the Alliance side (sorry, Horde - I just don't have any easy equivalents for Stocks or Deadmines - Wailing Caverns is a bear of an instance) low-level instances and using the fastest route for getting linked toons to level 40.
Levels 1-10: questing
Unfortunately, there are no quick ways to get to level 10 other than questing. Simple enough, though - you'll get to level 5 before finishing even half of the starting area quests, and level 10 before you get to the dreaded Westfall/Darkshore/Loch Modan/Bloodmyst Isle areas. When you get to level 10, head to ...
Levels 10-15: Deadmines
With a level 80 priest, Deadmines is a breeze (well, I'm sure it'd be with any class). Each large "room" will be one pull - bubble yourself and proximity-aggro (run right on top of them, or close to it) everything in each room, don't stop, get to a part where you can line-of-sight everything around a corner, wait until those damn mages get to you, and then Holy Nova everything down. Yes, Holy Nova - at these low levels, Holy Nova is fast and easy (instant cast) and does sufficient damage to where you only need to spam it a couple of times (at Deadmines, literally two times to kill the elites). Once you are level 15 after two-three runs, go to:
Level 15-25: Stormwind Stockades
This instance can literally be completed in two pulls, though the second pull is a bear because the mobs knock you down. Keep yourself bubbled and run into each room up the first hallway, the room down the hall, and then go to the left into each room, keeping yourself bubbled the entire time. Do not attack anything, just keep yourself bubbled and use proximity-aggro again. When you get to the last big room with the three offshoot rooms, have your low level toons run to the big room while you gather those three offshoot rooms - in the last room, when all the mobs get there, maybe three Holy Novas should do it. Second pull just involves the last hallway - you will probably get frustrated because of all the knock-downs, but there's nothing you can do about those short of skipping that hallway altogether and just resetting the instance. Make sure you pick up the red bandana quest (from Lower City) as well as the kill quest outside of the instance from the guard guy - if you looted the letter off of Van Cleef in Deadmines and turned in that quest to the guy in the house beside the Stormwind temple, then you should have a third quest for Stocks that involves killing the main dude in there. There are two other quests for grabbing "the head of ____" (different prisoners), but one is in Darkshire and the other is in Lakeshire, and really .. takes too long to run to those. :-P
Anyway, when you get to level 25, you know where you're headed next ...
Level 25-40: Scarlet Monastery
Now, I was lucky enough on my priest to get my hands on the three-person Grand Black War Mammoth from Vault runs, so it was easy-peasy for me to run the low level toons to Southshore and even to Chillwind Point for those flight paths, and on to Scarlet Monastery. I read on WoWWiki that you can enter all four wings of SM at level 20, but I like to wait until 25 (because Stocks is so quick and easy, of course).
Start with Graveyard, of course - pull the entire upper courtyard in one pull, have your low level toons stand back by the instance portal, line-of-sight everything into the hallway next to the instance portal, and go to town with that Holy Nova. Second pull, of course, is the entire crypt area - have your low level toons wait right down the first staircase while you gather the entire crypt.
After one run of Graveyard, it's on to Library - same thing, have your low level toons stand back. This area can be done in maybe four pulls. Grab that key and it's on to Armory.
Armory is literally three pulls - easy as pie, just make sure to keep yourself bubbled, line-of-sight pull everything, and viola!
After the one run of Armory, it's time for Cathedral, which you will remain in until the low-level toons (who should, by now, be at 28-29ish) get to 39-40. With a priest, this instance can be done in three pulls (I've tried two pulls, but it didn't work - too many mobs.)
Bubble yourself, have the low level toons stand near the instance portal at the beginning of the hallway, and start gathering up the hallway's mobs as well as all of the mobs in the lower courtyard. I usually keep my finger poised on the Decursive box for easy removal of the mage mobs' frost bolt. When you've got them all (by proximity-aggro, again), run back to the beginning hallway and go just inside of it, in line of sight of the low level toons (have them move closer). As soon as those mages get in the doorway, Holy Nova everything (it'll take several of them).
Second pull is the upper courtyard - I have the low level toons stand on the other side of the bottom of the stairs from the waterfall part of the fountain, and I line-of-sight pull everything to the bottom of the waterfall.
Third pull is not a proximity-aggro pull. Instead, have your low level toons stand in the fountain on the top courtyard level. Bubble yourself and run straight in to Commander Mograine - as soon as you're within range, stick a DoT (I prefer SW:P) on him and run back out to the courtyard, JUST on the other side of the door (another line-of-sight pull). When you aggro the boss, you aggro everything in the cathedral building. Beats proximity-aggroing! Holy Nova that mess down, kill the two bosses quickly, run back out, reset, go back into Cathedral, and repeat your three pulls.
Congrats, that's your strategy for easily getting to level 40! (After that it becomes a little more tricky - as the mobs get higher in level, the pulls take longer - I prefer at this time to switch to my enhancement shaman and kill things as I come across them in groups, rather than big pulls like with the priest.) I do Zul'Farak next (I just do the quests from Gadgetgazan and that Carrot-on-a-Stick one) and then move over to Dire Maul east. I've gotten our main linked toons to 46 this way so far - will probably remain in Dire Maul until 52ish, and then move on to Stratholme/Scholomance.
On the Horde side, I have a level 58 deathknight that I've used to power-level our toons through quests (Blood elf world, we both hate the other Horde starting areas) to about level 15, and then we started our runs through Shadowfang Keep. They are at 21 now - I'm unsure of whether to stick with SFK until 25 (and then on to SM) or hop over to Blackfathom Deeps until 25. Any suggestions? SFK is taking a long time on each run, in my opinion - not nearly as efficient as Stocks or even Deadmines. Then again, the DK is 58 while my priest is 80 .. I'm sure that plays a big part.
Anyway, hope this helps for power-leveling some low level toons through some of the low level instances by using your priest. I also have a question -
How would a warlock do it? My main Horde toon will be a warlock, and I'm unsure of how to do big pulls on it - no bubbles or anything, and even a pet to get in the way. When your lock pet kills something without you touching it, there's no loot, right? Do you still get the xp for the kill? Could I just let a pet loose through a low level instance (like the Voidwalker), and when it has aggro of a lot of mobs, call it back to me and kill the mobs with that nifty aoe warlock fire spell? I'm unsure ...