I would imagine you'll want two separate focus keys and a uniform polymorph key so you can poly two targets at once. Other than that this should be pretty necessary (stolen from the Wiki):

Mage1:
/castsequence reset=24 Frost Nova,

Mage2:
/castsequence reset=24 ,Frost Nova

That way you can hit the same key and only cast one Frost Nova, then hit it again and the other mage will cast Frost Nova. Repeat when cooldown is up.