I'm not sure if you can set up FTL across multiple machines.
The F stands for Focus.
The T stands for Target.
And the L stands for Leader.

You can use any of these for your assist method quite easily.
However the Leader is no longer that great in pvp, since you cannot rearrange groups.

Target and Focus work just fine.

I'd lean towards Target, because Focus is useful for spell targeting, especially in pvp.

Have the slave/alt target the master/main.
And they can have their spells or castsequences like this:
/cast [target=targettarget] Spell.
So they always target the main, and cast at the target of their target (main's target).

FTL basically means you can switch to any toon, and lead from there.
And the others auto assist for the same target.

With a click castsequence, you'd just spam the buttons.
It's a lot easier with hardware or software broadcasting, so you're only mashing one key on one keyboard.

I'd really look at a software option.
Something that supports more then one PC.
Inner Space is my recommendation, I've been blown away by what it can do.
A year subscription is $36, and that allows up to 5 computers at once.
It has an automatic FTL set up, via its wizard.