Ok I have done some more experimenting and think I can explain it better.

As FTL is setup from the guide it uses the assist character command to pass targets to the non controlled clients.
I wanted to setup healing so that as my tank and rogue move through the hotbar 1, 2, 3, 4, 5, etc shooting off damage abilities my healer would also be healing from those same keypresses. I had thought to use the FTL system for that except a heal can't be cast on the assist command.

So then I thought to change the assist character to target character in the FTL macros as that will land a heal and should also cast damage abilites using the games pass through targeting (Cast on target's target).
That does work unless you are using a macro to cast the DPS ability. That is where my whole plan comes unstuck.

As a simple test make a macro like this on character2:
target character1
cast damage ability

If character1 has targeted an enemy the macro (used by character2) will target that enemy using pass through targeting but will not ever cast the damage ability.
This may be a bug in the game as logically this should work with "cast on target's target" enabled.

Anyway, what I want to do is be able to cast heals on the controlled client without messing with specific "target this character" macros.
It would work fine except for the above bug.

Sorry if this is confusing, it is late and I am off to bed!