Raid button configuration question

[Daemos]

Are w able to cast spells on the target's implied target..

I'ed like to be able to cast Countersong on a mob that is being targeted by someone (the MT or OT normal) without having to change my target.
Also would like to do this with other AAs we have just for fun (rather than having 3 version of Jester's cap on my buttons .)

Actually trying something tonight, will tell you if it works

[gm9]

assist
useability <your spell/AA>
target_previous