If you 2 HB keys, what I did for most of my RAF characters until 80 was I partied them, made the loot 'group loot' started one then waited a little bit, say a min or two and then started the other one. This keeps them in the same area for the most part and doing the same quests which in turn could make them go faster. Problems I've encountered is one can get ahead of another, looting problems (mainly with single quest items that each person in the party has to loot), and just looking quite suspicious with 2 characters running the same profile and hotspots which can make for a bad day if you're not watching them.
But like I said, it worked for the most part when one wasn't get stuck or d/cing and they stayed together. Faster leveling, especially with RAF, but you're better off just leveling each single until or if something ever gets updated or comes out to change all of this.
Good luck.
