The other option for this is to make the "infusion" similar to that of a Skillchain. (No not a Magic Burst - they rely on WS Skillchains).
This means any mage can do them - BLM, RDM, WHM, SCH... Basically 2x elemental properties (in skillchains) already make up an end result. Same concept here with Magic - 2x Spells make up a "Transfusion" which is basically an added effect of the 2x spells giving some sort of temporary enfeeble, and a damage bonus.
Example was something like..
Alderin casts Gravity on Shinryu.
Alderin casts Drain on Shinryu.
Transfusion: Demi! Shinryu takes 1200 damage and is effected by weighed down.
Or something simmilar of course. The other point to make is this should be doable even if a spell is resisted - Simply for the fact that a lot of NM's are immune to certain debuffs, so allowing for resistance will effectively eliminate the bonus.
