I actually don't mind this idea. Original!

But yeah agreed - give it to SCH as a job ability.

SCH uses Transfusion > Takes you to your magic list, you then select the 2x spells you wish to mix.

Macro's might look like this:

Code:
/ja "Transfusion" <me>
/ma "Silence" <t>
/ma "Aero" <t>
Then it casts Silent Wind or w/e.. Perhaps an equation for MP cost as something like-
Spell 1 MP + Spell 2 mp * 0.75.

Meaning it would cost 75% of the total MP of the spells if casted seperately.

The effect for Enfeeb + Elemental combination might be like a decreased resistance for the mob - so enfeebs stick, and an MAB for the Aero part. So the mob gets a heavier silence and aero with a MAB bonus casted on it.

Recast timer of like 2-5 mins..