You can do that very, very easily. Use Keyclone or AHK to pass the number keys from the main main account to the minimized/small windowed mage account.

Set up in game macros for each action you want the mage to perform.

1.
/follow main-account

2.
/assist main-account
/cast some-spell-here
/follow main-account

3.
/assist main-account
/castsequence reset=12 spell1, spell2, spell3

So when these macros are set up on the mage account, and Keyclone is setup and passing keys 1, 2, and 3, you would hit 1 on the main account, it gets passed to the mage account, the macro to /follow is on the action bar spot number 1, and the mage follows. Good luck, hope that helps you get a mental picture of what you need to do.