I have used my main (70 Shadow Priest) to level a number of alts on a different account. Since I have levelled so many characters to 70 now I know all the questlines and i'm not missing anything by skipping them. My usual progression in "Powerlevelling" as it were is.

1-15 in normal zones
15-28 in Stockades
28-40 in SM Armory

This works great, as a tip for more xp (possibly an exploit as such so sorry if i'm being naughty) if you have a priest this works great.

Take both your characters into an instance.. Leave your alt near the instance portal and Run round with your main and DoT a load of mobs (usually 3-5 at once) run back to your alt. Shield your alt and leave the instance with your main.

Your alt will take all the aggro now but at these levels will be able to survive with a lvl 70 Priest shield. Mobs will die from the DoT's but as your main is not in the instance your alt will get full XP from each mob as if they killed the mobs themselves.

When rested you can get around 1k xp per mob which ain't bad for a lvl 15 . It takes around 300 kills to level from 15-20 in stockades which takes around 2 hours or less if done fast.

As I say, could be naughty putting that down so sorry again!