You can go with a Target or Focus based system.
In the case of the Target system, target the tank and have the Mage assist you, so it is now targeting the tank too.

Then make your macros:
/cast [Target=TargetTarget] Whatever
Which can be abbreviated:
/cast [@TargetTarget] Whatever

Alternatively, go with a Focus approach.
Focus the tank.
And:
/cast [Target=FocusTarget] Whatever.