For frostnova you might want to use something like:

/castsequence reset=25 ,Frost Nova

and on the other character:

/castsequence reset=24 Frost Nova,

Bind them to the same key.

And for sheeping, I was thinking of this addon: RaidTarget, you simply assign let's say, cross and moon to the sheep targets and use a macro like:

/rt cross
/cast [exists, nodead, harm] Polymorph

Bind it to the same key on both mages, it should not cast polymorph if you don't assign the target. Not sure if this works though, haven't tried it myself yet.