For dual box combo, I'd considered Druid + Mage combo, as druid is more versatile. Main reason for recommending druid is that you could tank for PVE, and switch to boomkin for PVP. Based on my experience, two casters would be easier for PVP. It is hard to do the melee dance around your target while controlling your mage at the same time. And plus the occasional healing for the mage would be nice. That's just my take on this. Hope it helps.