In the mage group I dabbled with I the plan was to have one frost and the rest fire. So with two I would go one of each.

I always put 5 points in uninteruptable arcane missles first.

My reasoning is that one frost bolt will slow the mob allowing a second cast before they reach me. Fire for the rest as at lower levels as it seems to be the best dps.

When you start to AOE farm it all changes. Not sure how you would go targeting AOE spells remotely.