Here is how I do it with my warlocks...

I use a round robin key to assist my main. For each warlock I target the mob to be seduced or feared, then hit the assist key. Then I select the next target, hit assist key again. Now both warlocks have selected a different target.

All of my casts other than seduce and fear use [target=focus], focus being my main. So they never un-target their assist target. I can then hit fear and both cast fear on their own target.