I just started a draenei paladin/priest combo yesterday, they're up to level 15 in about 9 hours of /played time. I did spend some time idle (1-2 hours?) and tinkered with the setup using tips from here and figuring out the multibox application. Right now I'm having multibox just broadcast the numpad keys and I have a special set of hotbars on the second box bound to the numpad keys so when I hit them on the paladin's box, they get sent to the priest.

Num1 is the accept quest/trade/etc macro
Num2 is smite
Num3 is follow paladin
Num4 is mind blast
Num5 is SW:Pain
Num6 is heal paladin
Num8 is PW:Shield
Num9 is renew on paladin

I had just leveled a shaman to level 23 a few days before and it would appear that I am under-leveling relative to the quests I am working on (ie, I have to grind mobs a little bit in order to level up to open up quest opportunities) with the dualbox combo compared to the shaman. I figure if I leave the combo idle for a while and get some rest xp going, it won't be as much of a problem.

I chose a paladin instead of a warrior due to its 3-target taunt and better aoe capability. The priest will be able to be at range to heal and dps (shaman typically works best in melee for dps and druid is a bit too complicated for me).

I haven't had downtime between fights (maybe I need to push a little harder?), and this seems to handle well so far.