I've searched around for a couple threads on this question of mage dual boxing and specs, and there have been many different opinions thrown around..

What you are proposing (1 frost, 1 fire) has been referred to as alternately a Good Idea because of the different debuffs that can be achieved with 2 different specs, but also a Bad Idea because of the differences in mana usage and cooldown times.. so I'm not really sure what to believe. I'd like to get more opinions on different specs, but if none materialize I guess I will have to play around with specs to see what works.