Keyclone was/is a great product to start with.
The initial configuration, adding each window while it is active, was a bit of a pain in the ass.
But once you're up and running, anything you push goes to every window, unless you Blacklist the key.
Or you can push the Pause key to mute broadcasting.

IS Boxer has a wizard, which is far easier to use for the set up.
It has both Repeater (broadcasting of keys and mouse) or Keymaps (which are essentially triggered actions), and you'll sometimes want one or the other or both or neither on.
Not really that complex to pick one mode or another.
The actual wizard gets you a bunch, that are very easy to start with.
- Things like Slaves follow the Active/Main region... when you push the Follow Keybind.
- The current leader invites the slave/non-active characters... when you push the Invite Keybind.
- If you push 1, 2, 3, etc all the way up to = (which is the default bindings for the wow interface), the slaves first assist the main character and then do whatever is on those keys.

Which could be a straight spell dragged from each toons spell book.
No need for any macros of any sort.
But that could also be a castsequence macro, allowing you to mash one key and have some toons go straight Shadow Bolt spam, while another might be using Scorch, Scorch, Scorch, Pyroblast, Fireball, Fireball, Fireball.

It's very flexible.
And like HKN and KC has an amazing support level.