2 PCs, 3 Chars

I've set this up in the past, but haven't played in awhile.

Anyway. My setup is three characters on two PCs. 2 on main, 1 on secondary computer. Got them talking to each other (key broadcasting works for example) but failing to get my follow and assist to work on the secondary computer's character.

I would wager the problem has something to do with the variable keystrokes you're using. Try taking a look at the response Lax gave in this post. My guess is that you're encountering the same issue.

Probably right. =)

I get it, at a high level, I think... but where do I make the changes? Am I only making it for the second character set?

For the record, my advice is entirely based on a big assumption, so if I'm wrong please share your config...

As to what you need to change: basically, the 1st step of your Follow and Assist mapped keys. Presumably, this step contains a Variable Keystroke Action set to "Target Slot {SLOT}". If that's the case, then you need to change this to one of the options that Lax described in the post I linked. For example, let's say you named your characters in ISBoxer ToonA, ToonB, and ToonC. ToonA and ToonB are in one character set (meant to be launched on the first computer) and ToonC is in another. The 1st step in your Follow mapped key could contain a Variable Keystroke Action that does "Target {CHARACTER}".

Next, you'd have to add corresponding entries to the Variable Keystrokes tab (in the upper left pane): Target ToonA, Target ToonB, Target ToonC.

And lastly, follow the steps for assigning per-slot targeting keys--except instead of Target Slot 1 you're using Target ToonA, and so on.

Also make sure that both of your character sets are assigned to the key map that contains your Follow and Assist mapped keys.