The first 2 box characters I ran together were my 70 Druid and a lowbie Shaman. I powerleveled him to the mid 40s before soloing him. Pretty much just remember that you will need to hit every zone and quest that open up to you. Don't skip zones just because you don't like them. The quickest way to Powerlevel a newborn is to hit each and every starting and secondary starting zone for your faction.

I did Instance runs but only for the quest and maybe a second time for extra gear. I wouldn't suggest just doing instance runs along unless your secondary character has full rested, else the XP just isn't work the danger.

To me the ultimate Powerleveling comes from a Balance Druid. My moonkin was one shot killing mobs back to back using Moonfire up until the mobs go in the 30 range. This also worked for my 70 Frost Mage using Ice Lance. If you are using a class that doesn't have a powerful, spammable instance ranged attack on a short to non-existant cooldown it will slow you down. Also while using a 70 to powerlevel a lowbie the kill XP sucks major, but be sure to kill each and every mob even if you don't loot it. 10XP per kill isn't that much but if you can kill 20 of them in seconds just by spamming instants that one shot the mobs, it adds up.

Also unless you are needing to loot mobs for quest items, I would say don't bother for the most part if you have a 70 already. 70s can make plenty of gold to fun the lowbie so forget about looting trash until you get into the 30s and the chance of getting a blue drop that will actually bring in some cash goes up.