I tend to look at dual boxing in a couple of different ways.

1. What is a weakness in one character that is a strenght in another character.. My 23 fury warrior was great but his biggest weakness was no healing. My 22 feral druid was great in cat form but even in tank mode couldn't survive against 3 similar level mobs on this one quest in particular. (I managed to escape but the fight was a loser whichever way I tried it). I joined them together and basically turned my druid into a healbot, and cleared the quests that I'd struggled with in no time at all. I turned my warrior into a super powered paladin , healing abilities and rage bar - fantastic

2. What characters have abilities that complement each other. So for example I've got a 40 moonkin (who currently is more of an oomkin). I've got a shadowpriest i want to get to 40 (he's 38) as i think the moonkin and priest would have good synergy (even better from level 50 onwards when I get VT on the priest and will feed more mana to the moonkin). The rooting ability of the moonkin, the healing and mana back from the shadow priest, and the tankability of the moonkin seem to go well together.

I believe a mage and a priest are a good combination due to healing for a heavy dpser. A prot spec ao grinding pally combined with an aoe'ing mage may also be good.

I also consider the characters for end game. My priest at 70 could be turned in to a healer (or stay shadow for dps). My druid can be a tank a healer or dps. Add these two to my current seventies and I have a flexibile group for 5 mans.