assist the focus' focus target?

Lets say tank is Z played by someone else. Chars A, B, C are mine and A is my master.
B and C have my A in focus, and A has tank Z in focus. Can I in my assist macros on B and C target the focus of focus' target?

What I want to do is having only my char A put Z in its focus (my slaves still have my master A in focus), but make all three chars still shooting on the tank's target. Hope you understand what I mean. :)

Also another quick question.
Can a castsequence be reset every time a new target is selected?

no i dont think you can attack another persons focus target B and C would need to have Z as their focus and as for the cast sequence just do /castsequence reset=target.

If all of the toons are yours, you can have your focus (one of your toons) target it's focus, and then assist it.

But, for example, you focus the healer on the other team (arena). There is no way to detect who the healer has as their focus.