The only way I can think of is using some kind of /focus and /targetlasttarget thing.
Well having the macro only /assist when your main is not targeting someone, your alt will never be able to to anything. If it has no target to cast on it won't be able to seduce it. :P As you say "only assists my focus if I have no target or my target is dead" this macro will never being able to do anything.

I assume you want this as a CC right? How about usign something like this:
Code:
/assist maincharacter
/targetenemyplayer 
/cast Seduce
This will make your alt first target what you are targeting, then search for another char to cast Seduce on. If it finds a target, it will Seduce it.