How about the modifiers (help) and (harm)? Anybody know?

My guess is:

/cast (help) Intervene (harm) targetoftarget Intervene

Does that work?

I'm not sure if that's what you would use, but I'm pretty sure that you should replace the () with []. I'm not sure if it matters, but I always use brackets in my macros.

/cast [help] Intervene; [help, target=targettarget] Intervene

you have to use help in the second one because the "target=targettarget" is referring to the target of your target
so if you are targeting a bad guy when you use this the macro will check if you can help your current target which it cant if you are targeting the mob and since it cant help the mob it goes on to the next clause which checks if you can aide the mob's target

so if you use this on a friendly player it will help them, if you use it on a mob it will help the person receiving his attacks