Consider the following two scenarios:
1) One fire mage. Attack sequence is Pyroblast, fireball, fireblast, frost nova, open the range, scorch/wand/fireball until dead.
2) Two fire mages. Attack sequence is /assist Pyroblast, fireball, maybe fireblast, loot, NEXT!
The biggest difference you'll have is not having to open range because the mob will die about time it gets to you. Saving that extra time is where the kill speeds and mana savings come into play. Now, if you consider the same solo-vs-duo arrangement when pulling packs of mobs, being able to sheep 2 of a 3+ mob pack means you take a LOT less damage and can double your effective capabilities easily.
Two at once is most definitely better than doing them one at a time, especially if you will be mostly questing to level.
