As a new WoW dual-boxer, I've decided to prepend my harmful spells with

/assist Master
/cast [harm] [target=targettarget, harm]

my friendly spells with

/assist Master
/cast [help] [target=targettarget, help]

and my CC spells with

/assist Master
/cast [target=focus, exists, harm, nodead] [harm] [target=targettarget, harm]

This *should* allow me to cast friendly or harmful spells appropriately against my Master's target or my Master's target's target.

But this is a lot of macros to type.
Is there an addon that does something similar?
If not, would there be outside interest if I created one?
Is this even a recommended way to target (for 2-boxers)?