The best you'll get is to have the toon in question target their focus target.
And once they are doing so, acquire their target.

Toon A, wants to target the focus of their focus (Toon B).

Have Toon B target their focus.
/target [target=Focus]

Then have Toon A assist.
/target [target=FocusTarget]

But that only works, if you can make the focus of Toon A target their focus.
IE, if its your team, you can do it with a two step mapped key, or with two macros.
But you cannot make your arena opponent (Focus) target their own focus.